So then, brothers, stand firm, and hold the traditions which you were taught by us, whether by word, or by letter. (2Th 2:15 WEB)
OriginalChristianity.Net has been looking at where the Church is today doctrinally and in practice, and how the church has gotten from the exciting, powerful times of the New Testament to the church of today with many people professing Christianity, teaching scripture, even proclaiming manifestations of the spirit but also with many divisions and opposing viewpoints on many scriptural issues. And we read again in the verse above the Apostles’ charge to us to maintain the traditions that they handed to us. Traditions include beliefs and practices. We are charged to follow what they taught. And we have been warned in Acts 20 that after the Apostles’ passing that there would be people from both outside the church and inside that would twist the scriptures, drawing people away from the truth that Paul and the Apostles taught.
I know that after my departure fierce wolves will come in among you, not sparing the flock; and from among your own selves will arise men speaking twisted things, to draw away the disciples after them. (Act 20:29-30 ESV)
This verse is a prophecy that there would be people teaching things twisted from what the Apostles taught. The doctrine that we are looking at today has both extra-biblical thinking, for example, Jesus had two wills, and terms like homoousia, of the same substance, that are not in scripture, which are crucial to the Trinitarian argument.
This verse and other prophecies of things that would happen before Christ’s return also indicate that what Paul and the Apostles taught was not just for the Apostles’ time on earth. Their teaching was for the church age. As we are still awaiting the return of Christ, that includes us.
We are looking at a critical issue today, Unity vs Trinity, one person or three, what did the Apostles believe, teach, and practice? The answer lies in the fact that while “essential Christian Doctrine” today focuses on the Trinity, the Trinity was not present in original Christianity but scholars all agree that it was developed in the early centuries after Christ. So, the question is then what was the belief, if not the Trinity, about the natures of God the Father, Jesus Christ, and the holy spirit.
The answer is that, in fact, as is discussed in John 1 – The meaning of the Logos; The Slippery Slope of Applying Mathematical Precision to Language Expressions and, as it is stated at places as a historical reality, original Christianity started out Unitarian.
(Before proceeding any further it must be emphasized that we are talking about Christian or biblical Unitarianism and not Unitarian Universalism which is totally different and may not be Christian at all.)
Unitarian comes from the number one just as Trinity comes from the number three. It’s as simple as that, look it up. Unitarianism, simply put, is the belief that God is one. In contrast, Trinitarianism is the belief that God is three. Trinitarians say God is three persons while Unitarian just say God is one. Look at these references to what people believed in the first centuries about whether they believed that God is one or three persons:
“Justin nowhere asserts that the father, Son, and Spirit constitute one God, as became the custom in later ages, after the doctrine of the Trinity was fully matured. Strictly speaking, he was a Unitarian, as were the orthodox fathers generally of his time: that is, they believe the Son to be being really distinct from the Father, and inferior to him; which we take to be the very essence of Unitarianism.”
That is another reference to the fact that the “orthodox fathers” of Justin Martyr’s time were generally Unitarian, God is one, not three persons. Look at this:
Even after the elimination of Gnosticism, the church remained without any uniform Christology; the Trinitarians and the Unitarians continue to confront each other, the latter at the beginning of the third century still forming the large majority.
As it says above, Unitarians still formed a large majority at the beginning of the third century, indicating the Original Christian church started out Unitarian and continued that way for a least a couple of centuries.
And look at this Encyclopedia Americana reference to Unitarianism and its start:
“The most conspicuous point of departure from trinitarianism, the point usually emphasized, is the refusal to regard Jesus as “very God of very God” and the assertion that he was distinctly and unqualifiedly a human being. This belief in some form in a varying degree has accompanied Christianity from the beginning, at least as one of its forms.” 
There it is again; this belief [Unitarianism]… has accompanied Christianity from the beginning! That is the truth. But it’s also a truth that many theologians work to deny as evidenced in the next paragraph in the above-mentioned article. The Unitarianism of the original church is often denied today and it is barely conceded that the original church was not Trinitarian. Look at this careful wording as the article reads on:
It is usually conceded that even though it might not be correct to speak of Christianity during the first two or three centuries as being substantially Unitarian, it at least was not Trinitarian. It was this generally held belief that Christ was a man that Arius was trying to save in his conflict with Athanasius. It was this championship of Arius, of the human side of Jesus, that for centuries gave the name Arianism to any belief in the humanity of Jesus.
“It is usually conceded” alludes to the reluctance that the Trinitarian faction in the Church to admit the legacy of Unitarianism and that the Church took centuries of human debate over scripture that also included extra-biblical thinking and extrabiblical terms like homoousia (of the same substance) before it was declared official doctrine and more centuries to be put into its final form. “That it might not be correct” allows for a small chance for error, possibly because the Trinitarian contingent is widespread and regular with anti-Unitarian rhetoric. And it alludes to the fact that Unitarianism has never gone away, but has continued to be believed by some despite severe unscriptural practices like burning at the stake, the Inquisition, not to mention forfeiture of property, status, and position when churches and states were so intertwined. This quote also alludes to the fact that Unitarianism is most commonly called Arianism which was denounced when the deity of Christ and Trinitarianism began to replace Unitarian doctrine in the fourth century. I say began because Arius’ Unitarian teaching continued for some time after the council of Nicea.
So, while many Trinitarians do not want to concede that the church started out as Unitarian, it did and we are going to look at what they believed.
Tertullian was the first to introduce the concept of a trinity at the start of the third century. So, up until Tertullian the terms Trinitarianism, Unitarianism, and anti-trinitarianism didn’t apply so believers didn’t identify themselves as believing any of those doctrines, but in today’s terms, they were Unitarian.
Also, Trinitarians label Unitarianism as Arianism as if Arius invented it, but he didn’t. While what Arius taught completely about his viewpoint is unknown and we don’t know how much it resembles scriptural Unitarianism in its entirety Arius just tried to continue some of what had been received by most as correct doctrine since the Church began, that Jesus was a man, a very special man, inferior to God, and certainly not God himself.
Also, in regards to terminology, there is a bias in being known as anti-anything, For example, anti-abortion paints people as a negative while pro-life paints people in a positive. Is the other camp pro-choice or anti-fetal rights? While Trinitarians like to call Unitarians “anti-trinitarian” they weren’t anti-trinitarian because Trinitarianism didn’t exist so there wasn’t even anything to be “anti” against at that in the first century. If anything, it is Trinitarians that are anti-Unitarian because Unitarianism predates Trinitarianism. And that makes Anathasius, Constantine, the majority of the Nicene council, and every one up to now that is Trinitarian “anti-Unitarian”.
Once again, as Merriam-Webster defines a Unitarian as someone who believes that the Deity exists only in one person, then they, the original Christian believers were Unitarian. Unitarianism, though, has been seen in different forms.
Here’s another quote showing that Unitarianism came first, that it is the Apostles’ tradition:
“Unitarianism as a theological movement began much earlier in history; indeed it antedated trinitarianism by many decades. Christianity derives from Judaism, and Judaism was strictly Unitarian. The road which led from Jerusalem to Nicaea was scarcely a straight one. Fourth century trinitarianism did not reflect accurately early Christian teaching regarding the nature of God; it was on the contrary a deviation from this teaching. It developed therefore against constant Unitarian or at least anti-Trinitarian opposition.
What is Unitarianism? Compared to the Trinity, Unitarianism is simple and uncomplicated without the impossible to understand incongruities that the Trinity carries with it. The Father alone is God. Jesus was a special man, fathered by God, but just a man who took on the assignment of our salvation and died for our sins, rose from the dead, and is now seated at the right hand of God. As God is holy, and God is spirit, holy spirit is just God in action. Holy spirit is the term used when the power of God is communicated in creation and inspiration. It’s that simple.
As Judaism is and was strictly Unitarian, no Jew misunderstood King David when he wrote:
Create in me a clean heart, O God. Renew a right spirit within me. Don’t throw me from your presence, and don’t take your holy Spirit from me. (Psa 51:10-11 WEB)
David would have been shocked had someone said to him that the holy spirit he was talking about was another person in a triune Godhead! David understood that the holy spirit he had was his connection to God, placed upon him for inspiration and guidance. His request shows how God uses the holy spirit to form a relationship with him. He begs the father to not discard him and take his holy spirit away from him because he knows when his holy spirit is gone from him he is no longer in God’s presence. Remember that in the Old Testament holy spirit was placed upon select people whereas Christ made it available for all believers to receive the holy spirit in us. David’s predecessor, Saul, is an example of someone who lost his holy spirit in Old Testament times.
The original Christians believed that Jesus was subordinate to the father, not coequal, not coeternal, and not part of a Trinity! Only the Father is God eternal! Look at Jesus’ words:
This is eternal life, that they should know you, the only true God, and him whom you sent, Jesus Christ. (Joh 17:3 WEB)
Jesus called the Father the only True God. That’s Unitarian doctrine. Jesus didn’t say they were God together, he said the Father is the only true God.
Paul, likewise, wrote of one God, the Father, and one Lord (Master, ruler, the guy in charge while he may have a boss). Yahweh is God, while Jesus is Lord because Yahweh empowered him.
So, in this original Christian setting, who is the Father?
Hear, Israel: Yahweh is our God. Yahweh is one. You shall love Yahweh your God with all your heart, with all your soul, and with all your might. (Deu 6:4-5 WEB)
The Lord God is one. He is one person! He alone is God.
Yahweh… says: “I am Yahweh, who makes all things; who alone stretches out the heavens; who spreads out the earth by myself; (Isa 44:24b WEB)
Yahweh says he alone spread out creation. Alone refers to a single person. Myself refers to one person! Yahweh says “I” here and in so many places. (Yes, there are a few places where the pronoun “we” is used but even there the verb indicates singular, not plural persons acting.) Yahweh is referred to as a person, but only one person.
Who is this God the Father, Yahweh? Yahweh’s declaration of his name to the children of Israel speaks loudly to who God is and how hard he is to comprehend to us mere humans.
Now Moses was keeping the flock of Jethro, his father-in-law, the priest of Midian, and he led the flock to the back of the wilderness, and came to God’s mountain, to Horeb. Yahweh’s angel appeared to him in a flame of fire out of the middle of a bush. He looked, and behold, the bush burned with fire, and the bush was not consumed. Moses said, “I will go now, and see this great sight, why the bush is not burned.” When Yahweh saw that he came over to see, God called to him out of the middle of the bush, and said, “Moses! Moses!” He said, “Here I am.” He said, “Don’t come close. Take off your sandals, for the place you are standing on is holy ground.” Moreover he said, “I am the God of your father, the God of Abraham, the God of Isaac, and the God of Jacob.” Moses hid his face; for he was afraid to look at God. Yahweh said, “I have surely seen the affliction of my people who are in Egypt, and have heard their cry because of their taskmasters, for I know their sorrows. I have come down to deliver them out of the hand of the Egyptians, and to bring them up out of that land to a good and large land, to a land flowing with milk and honey; to the place of the Canaanite, the Hittite, the Amorite, the Perizzite, the Hivite, and the Jebusite. Now, behold, the cry of the children of Israel has come to me. Moreover I have seen the oppression with which the Egyptians oppress them. Come now therefore, and I will send you to Pharaoh, that you may bring my people, the children of Israel, out of Egypt.” Moses said to God, “Who am I, that I should go to Pharaoh, and that I should bring the children of Israel out of Egypt?” He said, “Certainly I will be with you. This will be the token to you, that I have sent you: when you have brought the people out of Egypt, you shall serve God on this mountain.” Moses said to God, “Behold, when I come to the children of Israel, and tell them, ‘The God of your fathers has sent me to you;’ and they ask me, ‘What is his name?’ What should I tell them?” God said to Moses, “I AM WHO I AM,” and he said, “You shall tell the children of Israel this: ‘I AM has sent me to you.’” (Exo 3:1-14 WEB)
God is not a person like you and me. God is so awesome that we cannot look at him in our weak state.
“I am that I am” as a name is so powerful and speaks more to who God is than calling him just a person like the rest of us.
God named himself YHWH to which the vowels were added and became Yahweh, as it says in the next verse.
God said moreover to Moses, “You shall tell the children of Israel this, ‘Yahweh, the God of your fathers, the God of Abraham, the God of Isaac, and the God of Jacob, has sent me to you.’ This is my name forever, and this is my memorial to all generations. (Exo 3:15 WEB)
Yahweh is the creator. Scripture starts with the declaration that in the beginning, God created the heavens and the earth. And as we read above (Isa 44:24) he did it alone.
In the beginning, God created the heavens and the earth. (Gen 1:1 WEB)
Now, in that creation, he created a lot of stuff, stars, worlds, etc. He also created angels, though there is little about this in scripture.
Thus the heavens and the earth were finished, and all the host of them. (Gen 2:1 ESV)
The host referred to above could mean all the stars, galaxies, planets, and such but it could also refer to angels. Angels aren’t Yahweh, they are created spirit beings with power and responsibilities.
God alone is God, and his being, his power is beyond us to measure.
But will God in very deed dwell on the earth? Behold, heaven and the heaven of heavens can’t contain you; how much less this house that I have built! (1Ki 8:27 WEB)
But our God is in the heavens. He does whatever he pleases. (Psa 115:3 WEB)
Great is our Lord, and mighty in power. His understanding is infinite. (Psa 147:5 WEB)
We can’t fathom his thought processes, his mind, his intelligence.
“For as the heavens are higher than the earth, so are my ways higher than your ways, and my thoughts than your thoughts. (Isa 55:9 WEB)
God speaks and things happen.
“For my thoughts are not your thoughts, and your ways are not my ways,” says Yahweh. “For as the heavens are higher than the earth, so are my ways higher than your ways, and my thoughts than your thoughts. For as the rain comes down and the snow from the sky, and doesn’t return there, but waters the earth, and makes it grow and bud, and gives seed to the sower and bread to the eater; so is my word that goes out of my mouth: it will not return to me void, but it will accomplish that which I please, and it will prosper in the thing I sent it to do. (Isa 55:8-11 WEB)
God’s word, his planning, his intelligence, his Logos, gets spoken and starts to work at the same time.
Now, while this does say that God’s ways are higher than our ways and His thoughts are so much higher than our thoughts that we can’t understand them, we can understand the things that he tells us.
The secret things belong to Yahweh our God; but the things that are revealed belong to us and to our children forever, that we may do all the words of this law. (Deu 29:29 WEB)
Yes, God is this incredible reality beyond our comprehension, but we can understand what he tells us because he knows what we will be able to understand and he wants us to know some things. There is one and only way to know something about Yahweh; what he has revealed to us, the other things are secret.
In the physical world, we have a grasp of a lot of things and can experiment to test to see how things are. That’s science. That’s inductive reasoning, and we can use it in the physical world to learn things.
Beyond what he has revealed we cannot be sure of how God works or his nature because he is so beyond our understanding. Beyond what is revealed cannot be determined because of the uncertainty of trying to figure out something beyond measure and beyond our comprehension. Yes, we know some things he has revealed in his word, but there is so much that we don’t know that we cannot make determinations about what is not revealed.
But as it is written, “Things which an eye didn’t see, and an ear didn’t hear, which didn’t enter into the heart of man, these God has prepared for those who love him.” But to us, God revealed them through the Spirit. For the Spirit searches all things, yes, the deep things of God. For who among men knows the things of a man, except the spirit of the man, which is in him? Even so, no one knows the things of God, except God’s Spirit. But we received, not the spirit of the world, but the Spirit which is from God, that we might know the things that were freely given to us by God. Which things also we speak, not in words which man’s wisdom teaches, but which the Holy Spirit teaches, comparing spiritual things with spiritual things. Now the natural man doesn’t receive the things of God’s Spirit, for they are foolishness to him, and he can’t know them, because they are spiritually discerned. (1Co 2:9-14 WEB)
The words in these verses are all about the futility of trying to understand the nature and workings of God through human means like philosophizing and making hypotheses and testing them with inductive logic, incorporated with deductive logic and a priori assumptions, and the like which is how the doctrine of the Trinity was constructed. Men can’t understand God with philosophy, human wisdom. That’s what the verse above says.
“God revealed them through the Spirit” is how we know things about God! Natural man can’t know them because they are spiritually discerned.
We know the things freely given to us by God via the spirit. That is what scripture says. It also says we can’t know them with man’s wisdom. Our philosophies (wisdom of men) can teach a lot of things but not things of the spirit of God. If we know a truth from God it has been spiritually discerned either by prophecy in scripture or personal revelation (refers more to things happening in a person’s life).
Jesus Christ in Unitarianism is a man who accepted the commission to be the Messiah. Jesus clearly taught that by himself he did not have God’s power, but he was just following God the Father in what he did. Therefore, he is God’s appointed agent, his Messiah, his Christ. Here Yahweh’s Messiah talks about his relationship to the Father:
Jesus therefore answered them, “Most certainly, I tell you, the Son can do nothing of himself, but what he sees the Father doing. For whatever things he does, these the Son also does likewise. (Joh 5:19 WEB)
Jesus can do nothing of himself! God can do things of himself, Jesus can’t. Jesus is subordinate.
Jesus was a man, a seed (offspring) of Eve, who was promised to rectify the debacle in the Garden of Eden when the Devil deceived Eve into sinning and Adam followed suit.
And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel. (Gen 3:15 KJV)
Jesus was special in that he was the second Adam, and born by the power of the holy spirit.
Now the birth of Jesus Christ was on this wise: When as his mother Mary was espoused to Joseph, before they came together, she was found with child of the Holy Ghost. (Mat 1:18 KJV)
And so it is written, The first man Adam was made a living soul; the last Adam was made a quickening spirit. (1Co 15:45 KJV)
Jesus, being a man, didn’t know everything, he was limited in certain ways and had to grow as all people do.
And Jesus increased in wisdom and stature, and in favor with God and men. (Luk 2:52 WEB)
But no one knows of that day and hour, not even the angels of heaven, but my Father only. (Mat 24:36 WEB)
But Jesus fulfilled his mission! And Jesus was elevated to God’s second in command when he fulfilled that mission and was raised from the dead. All power is given to him, except it is known that it is God the Father that gives him this power and is truly God over all, even over Christ.
This Jesus God raised up, to which we all are witnesses. Being therefore exalted by the right hand of God, and having received from the Father the promise of the Holy Spirit, he has poured out this, which you now see and hear. (Act 2:32-33 WEB)
Still in all this Christ is subject to the Father as it says in the following verse.
The last enemy that will be abolished is death. For, “He put all things in subjection under his feet.” But when he says, “All things are put in subjection”, it is evident that he is excepted who subjected all things to him. When all things have been subjected to him, then the Son will also himself be subjected to him who subjected all things to him, that God may be all in all. (1Co 15:26-28 WEB)
The above verse says that God the Father put Christ over all things except the Father. Everywhere and at all times Jesus was and is subordinate to the Father.
God is holy spirit because God is holy and God is spirit. That is just who he is. Whenever holy spirit is used in scripture it refers to God inspiring and empowering others in the gift he gives to us.
For I am Yahweh your God. Sanctify yourselves therefore, and be holy; for I am holy. (Lev 11:44a WEB)
God is spirit…” (Joh 4:24a WEB)
There it is: God is holy and God is spirit; God is holy spirit. When we believe, Yahweh gives us some of his holy spirit;
Peter said to them, “Repent, and be baptized, every one of you, in the name of Jesus Christ for the forgiveness of sins, and you will receive the gift of the Holy Spirit. (Act 2:38 WEB)
Yahweh communicates with the spirit he puts in us;
The Spirit himself testifies with our spirit that we are children of God; (Rom 8:16 WEB)
Yahweh empowers us through his spirit;
that he would grant you, according to the riches of his glory, that you may be strengthened with power through his Spirit in the inward man; (Eph 3:16 WEB)
Yahweh enables different gifts to people through holy spirit;
Having gifts differing according to the grace that was given to us, if prophecy, let’s prophesy according to the proportion of our faith; or service, let’s give ourselves to service; or he who teaches, to his teaching; or he who exhorts, to his exhorting; he who gives, let him do it with generosity; he who rules, with diligence; he who shows mercy, with cheerfulness. (Rom 12:6-8 WEB)
That is the Apostles’ tradition, what they handed down to us. It’s what they put in practice. It is what we are charged to follow in 2Tim 2:15 and other places.
Compared to the incomprehensible doctrine of the Trinity Unitarianism is a welcome breeze of fresh air. It is much simpler. It is understandable. It is freeing not to be bound under the weight of the mind-boggling complexities of the Trinity, a man-made doctrine that took centuries to formulate, and then the Roman Church took extreme measure to enforce compliance to point that it is still embedded in so many Christian lives today.
Be free from the ravages of this incomprehensible doctrine. If the Father is God and the Son is God and the Spirit is God why aren’t they equal to each other? In Philosophy that is called incongruence. It doesn’t work. Where in scripture does it say that Jesus had two wills? Yet that is what the doctrine of the Trinity demands because Scripture says that God cannot be tempted but Jesus was tempted in all ways. The incongruities go on and on. Be like the original Christians, accept the Unitarian view, God the Father alone is God, Jesus Christ, a man, was the son of God, and holy spirit refers to God in action with us as he gives the gift of holy spirit to men and interacts with them.
Always remember that we are charged by the Apostles to follow their tradition.
So then, brothers, stand firm, and hold the traditions which you were taught by us, whether by word, or by letter. (2Th 2:15 WEB)
In closing, I want to ask an interesting question. What do Henry W Longfellow, Oliver Wendell Holmes, Ralph Waldo Emerson, William Cullen Bryant, Nathaniel Hawthorne, Louisa May Alcott, Thomas Jefferson, John Adams, John Q Adams, Millard Fillmore, William H Taft, and Daniel Webster all have in common? The answer is they were all Unitarians! So were Isaac Newton and John Milton. These are just some of the many notable people that have realized the truth in this area. This is not a doctrine believed by just unlearned and ignorant men.
It’s time for all who call upon the name of the Lord to embrace Unitarianism again.
Further Reading on Christian Unitarianism (including links to resources available online)
Encyclopedia Americana, 1920 Edition, Vol XXVII, p. O301 available online at https://ia800305.us.archive.org/33/items/encyclopediaame23unkngoog/encyclopediaame23unkngoog.pdf. This article in this century old Encyclopedia Americana is the better part of 10 pages long and reflects that Unitarianism was more known then. Find the topic Unitarianism
Statement of Reasons For Not Believing The Doctrines of the Trinitarians, Andrews Norton, London, 1846
The Doctrine of the Trinity, Anthony F. Buzzard and Charles F. Hunting, Atlanta Bible College and Restoration Fellowship, 1990
The Elements of Unitarianism, George Chryssides, Element Books, Dorset, 1998
The Epic of Unitarianism, David B. Parke, Skinner House Books, Boston, 1957
The History of The Doctrine of the Trinity The True Scriptural Picture, http://www.antipas.org/books/trinity/trinity1.html
The Trinity: True Or False? Peter J. Southgate, Dawn Book Supply, 1995, A Christadelphian book available at https://www.the1way.net/wp-content/uploads/2015/03/THE-TRINITY-true-or-false-2nd-edition.pdf
The Racovian Catechism, available at http://thehumanjesus.org/media/pdf/The_Racovian_Catechism.pdf
The Two Treatises of Servetus on the Trinity, Michael Serveto, Translated by Earl Morse Wilbur, Wipf & Stock, Eugene, Published 2013
One God & One Lord : Reconsidering the Cornerstone of Christian Faith, Mark H Graeser, John A. Lynn, John W Schoenheit, Christian Educational Services, 2000
One God Over All (Class), Living Hope International Ministries, available at https://lhim.org/lhim-class/?id=84
 Much of the groundwork for this has already been laid. In 03.25.1 The Great Councils Continued, Finishing the Development of Trinitarian Doctrine and Related Issues; Over 500 Years of Debate on the Nature of Christ and the Trinity we see that the Trinity was a slowly developed doctrine based on philosophical arguments that started early in the second century and was initially put in place at the Council of Nicea in 325 AD. That article explains how beliefs changed from original Christianity slowly and all of the changes to fully develop the Trinity weren’t fully in place until the seventh century A.D. It also explains that in the process of developing this doctrine, extra-biblical terms and thinking processes were used to formulate it.
Also, the co-equality and co-eternality of the persons in the Trinity are building blocks of Trinitarian doctrine. In John 1 – The meaning of the Logos; The Slippery Slope of Applying Mathematical Precision to Language Expressions. the true meaning of the Logos is presented. And it is shown that this section of John’s gospel does not substantiate the Trinitarian view. Not only that, it is recognized and shown in the article that originally Christianity was not Trinitarian, but was actually Unitarian and this is written in sources such as:
The Encyclopaedia Britannica Vol.23 : Internet Archive p.963, says “the Trinitarians and the Unitarians continued to confront each other, the latter at the beginning of the 3rd century still forming the large majority”.
Furthermore, it is impossible to prove the trinity without the a priori assumption of a triune God. Only Trinitarian Christians believe in a triune God. Despite some imaginative research into Jewish writings, look at any Jewish site for support of the Trinity and you won’t find any. The original Apostles were all Jewish and believed likewise. Look at this statement on the Jewish encyclopedia.
(Despite arguments by Trinitarians purporting that the Jews believed in the Trinity),” The controversies between the Christians and the Jews concerning the Trinity centered for the most part about the problem whether the writers of the Old Testament bore witness to it or not, the Jews naturally rejecting every proof brought forward by their opponents.” (TRINITY: https://www.jewishencyclopedia.com/articles/14519-trinity).
According to Supplement to Trinity, Judaic and Islamic Objections, https://plato.stanford.edu/entries/trinity/judaic-islamic-trinity.html, it is generally accepted that neither Jews nor Muslims believe in a triune God. As the number of people believing in the trinity is not even a majority of persons this a priori assumption has poor justification.
Much of this including a priori assumptions is also discussed in Philosophy in Christianity – Welcome Addition or Intrusion of Worldly Reasoning? A priori comes from the Latin meaning “before”. A priori claims are things that you can base your argument on because they are self-evident. You don’t have to prove these claims because everyone knows that they are true, or at least that is the claim of the philosopher. In math, all angles in a triangle add up to 180 degrees. When you are arguing math, you don’t have to prove that. Or that 1+1=2. In your argument, you can assume them to be true. When you use a priori claims you are starting with an assumption that something is true. That is part of this process.
Do all math knowledgeable Christians, Jews, Muslims, and every other kind of religion all believe that the angles in a triangle add up to 180°? There is no dispute, no matter the religion on that claim. But, do all Christians, Jews, Muslims, and every other kind of religion all believe that God is triune? Of course, the answer is no, they do not. While some Christian groups have attempted to “prove” the existence of Trinitarian ideology in Jewish writings, there are no Jews or Muslims that believe in a triune God. That makes using the existence of a triune God as an a priori claim not justified. The original Apostles were all Jews and didn’t believe in the Trinity.
Additionally, in these and other articles, scripture verses that are used to preach the Trinity are shown that they don’t actually prove the Trinity as it is defined. Part of the reason for this that some verses are mistranslated to appear to substantiate the Trinity when they do not. For example, things created on account of Jesus Christ are not the same as things created by Jesus Christ. That all things were written on account of Jesus Christ is what is written, and so forth.
 The Church of the First Three Centuries, Alva Lamson D.D., Horace Fuller, Boston, 1880, p.80 available at https://www.google.com/books/edition/The_Church_of_the_First_Three_Centuries/i21QAQAAMAAJ?hl=en&gbpv=1&printsec=frontcover
 Encyclopedia Americana, Vol 23, page 301.
 Encyclopedia Americana, 1920 Edition, Vol XXVII, p. O301 available online at https://ia800305.us.archive.org/33/items/encyclopediaame23unkngoog/encyclopediaame23unkngoog.pdf. This article in the Encyclopedia Americana is the better part of 10 pages long. In a much more recent addition of the World Book Encyclopedia, the entry on Unitarianism was a long paragraph, a fraction of a page. That again demonstrates that the prominence of Unitarianism, the original belief of Judaism and its successor Christianity, has been slowly hidden over time. In searching the library catalog of my local library, there were no whole books on Unitarianism, and there were three books that had small references to the subject while a century ago Unitarianism was still a huge topic as demonstrated by the size of the entry into this encyclopedia that is about a century old!
 Encyclopedia Americana, volume 27, 1956, P. 2941, quoted in The Doctrine of the Trinity, Anthony F. Buzzard and Charles F. Hunting, Atlanta Bible College and Restoration Fellowship, 1990 P. 19
 Encyclopedia Americana, 1920 Edition, Vol XXVII, p. O301 available online at https://ia800305.us.archive.org/33/items/encyclopediaame23unkngoog/encyclopediaame23unkngoog.pdf
 The Doctrine of the Trinity, Anthony F. Buzzard and Charles F. Hunting, Atlanta Bible College and Restoration Fellowship, 1990 P. 112-113