Eusebius had been in Caesarea Maritima when Agapius was bishop and become friendly with Pamphilus of Caesarea, with whom he seems to have studied the text of the Bible, with the aid of Origen's Hexapla and commentaries collected by Pamphilus, in an attempt to prepare a correct version.

Pamphilus had been imprisoned in 307 but Eusebius continued their project.

The resulting defense of Origen, in which they had collaborated, had been finished by Eusebius after the death of Pamphilus in 309 and sent to the martyrs in the mines of Phaeno in Egypt.

Eusebius then seems to have gone to Tyre and later to Egypt, where he first suffered persecution.

Next heard of as bishop of Caesarea Maritima, he had succeeded Agapius, whose time of office is not known, but Eusebius must have become bishop soon after 313.

Nothing is known about the early years of his tenure.

Eusebius recounts early Christian history in his Chronicle, which, as preserved extends to the year 325.

Describing the development of the church in his Ecclesiastical History, which he begins around this time, he takes a historical approach in his attempt to explain the association of Christianity with the Roman Empire.