Eusebius is recognized as the father of Christian Church history. Living from about 260 – 339A.D he was the first to undertake the task of recording the growth of Christianity from a small Jewish sect in Jerusalem to a preeminent religion in the then known world. As he is the only person whose writings have survived from this early period his work is constantly referred to in writings on ancient Christianity.
Eusebius himself defines the goal of the work:
1. It is my purpose to write an account of the successions of the holy apostles, as well as of the times which have elapsed from the days of our Saviour to our own; and to relate the many important events which are said to have occurred in the history of the Church; and to mention those who have governed and presided over the Church in the most prominent parishes, and those who in each generation have proclaimed the divine word either orally or in writing. 
While this is a lofty goal Eusebius himself acknowledges that there are problems getting exact information regarding all the events. As he goes through his history point by point he will use whatever reference material he has, acknowledging that some materials have already been lost. He periodically will give facts that he has gathered, but give an opinion on the credibility and validity of the facts.
Nevertheless he is one of the greatest sources of information regarding early Christianity. In his history are records of documents that exist nowhere else. And he demonstrates that even those many centuries ago people were trying to resolve biblical discrepancies. For example, he devotes a chapter on the problems of the genealogy of Jesus in the book of Matthew. 
Eusebius was a participant in formulating the Nicene Creed which formally pronounced the deity of Christ. The first major point in his history that Eusebius makes is a dissertation on the deity of Christ.  Eusebius begins his history of Christ with his role as God the Son before history began. His opening words are more doctrinal treatise than they are the recollection of historical facts:
Since in Christ there is a twofold nature, and the one — in so far as he is thought of as God — resembles the head of the body, while the other may be compared with the feet — in so far as he, for the sake of our salvation, put on human nature with the same passions as our own — the following work will be complete only if we begin with the chief and lordliest events of all his history. In this way will the antiquity and divinity of Christianity be shown to those who suppose it of recent and foreign origin, and imagine that it appeared only yesterday.
This bold statement is well beyond anything found in the Bible, even the preamble of the Gospel of John. While John speaks of the Word made flesh, an often used phrase by Trinitarians to establish the deity of Christ, Eusebius plainly states that somehow the divinity of Christ had been long established. In fact, he argues in the above paragraph that he is stating Christ as God before time began to show those who suppose the divinity of Christ to be” of recent and foreign origin and imagine that it appeared only yesterday.” This is obvious propaganda. It is also in spite of the fact that despite the ratification of the Nicene creed the position on the Deity of Christ was so controversial that it would be reversed and re-reversed etc until the doctrine of the trinity would come out decades later. Notice that neither God the Holy Spirit nor the “Trinity” are included in the arguments presented by Eusebius, only the Deity of Christ, the current controversial topic of the day.
This weakens his validity as a writer, as he is addressing doctrinal issues on the very first point of his history. Not only is he addressing doctrinal issues but he is addressing then contemporary points about the issue. This reeks of revisionist history where a historian gives their account of history to coincide with contemporary beliefs.
Eusebius also makes doctrinal statements on other issues. “Eusebius held to the position of the Early Church before Augustine, that men were sinners by their own free choice and not by the necessity of their natures.” This is in opposition to later doctrine developed by Augustine and others that men were sinners by nature, original sin had made them so.
The net result of Eusebius’ writing style to include these doctrinal elements is that it becomes obvious that one of the purposes of his writing is be propaganda for the orthodoxy of his day.
Eusebius has been for the most part highly honored and well-received. As said above, he is the only source for some elements in Church history. Most scholars, I would say, place a high degree of reliability on his writings, while watching out for his shortcomings.
There are, however, critics who say that Eusebius was a poor historian, and theologian. “Edward Gibbon (18th century historian) dismissed his testimony on the number of martyrs and impugned his honesty by referring to a passage in the abbreviated version of the Martyrs of Palestine attached to the Ecclesiastical History” Jacob Burckhardt (19th century cultural historian) dismissed Eusebus as “the first thoroughly dishonest historian of antiquity”. 
Eusebius’s Ecclesiastical History is an invaluable resource. But Eusebius’ bias and other shortcomings must be watched out for. Nevertheless it is impossible to get better insight into the growth of ancient, primitive Christianity without consulting this important book.
 Point 1 of The Plan of The Work, found at http://www.newadvent.org/fathers/250101.htm
 ibid, Chapter 7. The Alleged Discrepancy in the Gospels in regard to the Genealogy of Christ.
 ibid, “Chapter 2. Summary View of the Pre-existence and Divinity of Our Saviour and Lord Jesus Christ.”
© copyright 2010 Mark W Smith, all rights reserved.