Eusebius in a modern imagining Eusebius of Caesarea (; , ''Eusébios tés Kaisareías'';  260/265 – 339/340), also known as Eusebius Pamphili (from the , was a historian of Christianity, exegete, and Christian polemicist. He became the bishop of Caesarea Maritima about 314 AD. Together with Pamphilus, he was a scholar of the Biblical canon and is regarded as an extremely learned Christian of his time. He wrote ''Demonstrations of the Gospel'', ''Preparations for the Gospel'', and ''On Discrepancies between the Gospels'', studies of the Biblical text. As "Father of Church History" (not to be confused with the title of Church Father), he produced the ''Ecclesiastical History'', ''On the Life of Pamphilus'', the ''Chronicle'' and ''On the Martyrs''. He also produced a biographical work on the first Christian Emperor, Constantine the Great, who ruled between 306 and 337 AD.

During the Council of Antioch, which was convened in 325 and held shortly before the First Ecumenical Council in the Bythnian city of Nicaea, he was excommunicated for subscribing to the heresy of Arius.

The debate raged over whether Christ is of the "same substance" (homoousion), or "similar substance" (homoiousian) to the Father. This was no exercise in semantics, for if Christ is of the 'same substance', or 'essence' as the Father, then this testifies to His divinity. On the other hand, if Christ is of 'similar substance' to the Father – as Arius believed, then this testifies that Christ had a beginning in time, is therefore subordinate to the Father and is little more than a created being. While the Greek fathers of the church applied the Aristotelian term ''ousia'' to the divine essence of God, it translated into Latin as ''essentia,'' or ''substantia'' (substance). The Greek term ''homoousios'' was consequently translated into Latin as ''coessentialis'' or ''consubstantialis,'' which in English translates as ''coessential'' and ''consubstantial.'' Thus the test for heresy at Nicaea was based upon what eventually became the orthodox confession that the Father and Son are ''consubstantial (homoousios);'' which is to say that they are of ''identical substance,'' or, as the Greeks would say, ''essence'' with each other – which then professed the emerging Trinitarian faith, as the object of Nicaea was to preserve the divinity of Christ, by stating that the divinity of the Son is consubstantial with the Father.

As the dogma of the Trinity came to be refined in the succeeding centuries, this led to ongoing problems associated with Arius' original accusation of modalism, which he had leveled at Alexander; for prominent 19th century church historian Adolf von Harnack says of Latin theologian Augustine (354–430), celebrated "Doctor" of the Catholic Church, and who has been called the most significant Christian theologian “since New Testament times," that "''Augustine only gets beyond modalism by the mere assertion that he does not wish to be a Modalist.'''" Drawing upon previous theologians, such as Tertullian and Ambrose of Milan (who baptized him in 387), Augustine was instrumental in describing the Filioqe (the procession of the Holy Spirit) in De Trinitate(the Trinity), which saw some Latin churches begin to adopt the term in the 6th century, which began a schism between the Latin and Greek churches.

The main contenders for the faith at Nicaea were Athanasius of Alexandria, and Arius, who vehemently opposed each other. Drawing from " ... the language and thoughts of Hellenistic [Greek] metaphysics", and " ... employing the terms ''substance'' in the Nicene Creed and ''consubsantial'' in the Chalcedonian Creed, the church fathers were tapping into Neoplatonism." Arius was no exception, for according to Baptist theolgian Roger Olssen:
"In the deep background of the clash between Arius and Alexander over the nature of the Logos lay Greek philosophy. It is something both had in common, even if they interpreted and applied it differently. Both sides of the conflict simply assumed that divinity is ontologically perfect in such a way that any change at all is impossible for and improper to attribute to it. Thus God, being divine and therefore absolutely perfect, cannot experience change because change is always to change either for the better or for the worse, and in either case God would not be God if he could change. Absolute static perfection – including ''apatheia,'' or impassibility (passionlessness) – is the nature of God according to Greek thought, and nearly all Christian theologians came to agree with this . . . God's immutability and impassibility, then, became chief attributes of God in Christian theology, and Arius and his followers exploited the argument that if Jesus Christ is the incarnation of the Logos and if the Logos is divine in the same sense that God the Father is divine, then God's nature would be changed by the human life of Jesus in time and God would have suffered with him. But that is impossible. Therefore the Logos who became incarnate in Jesus Christ must not be fully divine but rather must [according to Arius] be a great and exalted creature."
The first Christian Emperor, Constantine the Great, had converted to Christianity directly after the Battle of the Milvian Bridge in 312 A.D, after reputedly seeing a flaming cross in the sky which was emblazoned with the words "''In Hoc Signo Vinces";'' which translates to ''"In this Sign Conquer."'' According to Eusebius, in the evening before the battle Constantine went to sleep unsure what this sign could mean, but then had a dream in which Christ appeared to him, and commanded him to make a likeness of the sign which he had seen in the heavens, as it would protect him in all future engagements with his enemies. The next day, Constantine promised the Christian god that if his army were to win the battle, he would adopt the Christian religion - which is precisely what happened. In the June, 2002 edition of the 'Church History' journal, Pier Beatrice reports that Eusebius testified that the word ''homoousios'' (consubstantial) " ... was inserted in the Nicene Creed solely by the personal order of Constantine."
"According to Eusebius of Caesarea, the word ''homoousios'' was inserted in the Nicene Creed solely by the personal order of Constantine''.'' But this statement is highly problematic. It is very difficult to explain the seeming paradoxical fact that this word, along with the explanation given by Constantine, was accepted by the "Arian" Eusebius, wheras it has left no traces at all in the works of his opponents, the leaders of the anti-Arian party such as Alexander of Alexandria, Ossius of Cordova, Marcellus of Ancyra, and Eustathius of Antioch,who are usually considered Constantine's theological advisers and the strongest supporters of the council. Neither before nor during Constantine's time is there any evidence of a normal, well-established Christian use of the term ''homoousios'' in its strictly Trinitarian meaning.

Having once excluded any relationship of the Nicene ''homoousios'' with the Christian tradition, it becomes legitimate to propose a new explanation, based on an analysis of two pagan documents which have so far never been taken into account. The main theses of this paper is that ''homoousios'' came straight from Constantine's Hermetic background. As can be clearly seen in the ''Poimandres,'' and even more clearly in an inscription mentioned exclusively in the ''Theosophia,'' in the theological language of Egyptian paganism the word ''homoousios'' meant that the Nous-Father and the Logos-Son, who are two distinct beings, share the same perfection of the divine nature."
Although Eusebius' works are regarded as giving insight into the history of the early church, he was not without prejudice, especially in regard to the Jews, for while "Eusebius indeed blames the Jews for the crucifxion of Jesus, but he nevertheless also states that forgiveness can be granted even for this sin and that the Jews can receive salvation." Nor can his works be trusted to be from subjectivism, for some scholars believe that "Eusebius is a notoriously unreliable historian, and so anything he reports should be critically scrutinized." This is especially true of his '''Life of Constantine''', which he wrote as an eulogy shortly after the emperor's death in 337 A.D, and which is "Often maligned for perceived factual errors, deemed by some so hopelessly flawed that it cannot be the work of Eusebius at all." Yet others see him as a "Constantinian flunky," for as a trusted adviser to Constantine, it was politically expedient for him to present Constantine in the best light as possible. Never recognized as a saint, the likely reason for this is that traditional sources view Arius finding support " ... from Eusebius of Nicomedia, and our Eusebius, who by that time was bishop of Caesarea." Provided by Wikipedia

by Eusebius
Published 1720
Typis academicis. Curâ Cornelii Crownfield. Impensis executorum Joannis Nicholson nuperi bibliopolæ: & prostant venales apud Robertum Knaplock & Danielem Midwinter bibliopolas Londinenses

by Eusebius
Published 1709
printed by J.M. for Awnsham and John Churchill, at the Black Swan in Pater-Noster-Row

by Eusebius
Published 1703
printed for George Sawbridge at the Three Flower de-Luces in Little Britain
Subjects: '; ...Eusebius / of Caesarea, Bishop of Caesarea / ca. 260-ca. 340...

by Eusebius
Published 1720
typis academicis. Curâ Cornelii Crownfield. Impensis executorum Joannis Nicholson: & prostant venales apud Robertum Knaplock & Danielem Midwinter bibliopolas Londinenses

by Doedel, Eusebius
Published 2000
Springer New York

by Regius, Eusebius
Published 1736
printed for J. Roberts, near the Oxford-Arms in Warwick-Lane

by Smyth, John Eusebius
Published 1730
printed by S. Powell, for George Risk, at the Shakespear's Head, George Ewing, at the Angel and Bible, and, William Smith, at the Hercules, Booksellers in Dame's-Street

by Lotz, Johann Friedrich Eusebius
Published 1820

by Lotz, Johann Friedrich Eusebius
Published 1821
Bey J.J. Palm, und E. Enke

by Lotz, Johann Friedrich Eusebius
Published 1821
J.J. Palm und E. Enke

by Walker, Clement
Published 1651
Other Authors: '; ...Andrews, Eusebius...

by Newnan, John
Published 1793
Printed by Parry Hall, no 149, Chesnut Street, between Fourth and Fifth Streets
Other Authors: '; ...McCorkle, Samuel Eusebius...