At the Council of Nicæa, which met pursuant to an imperial summons in the year 325 A.D., Eusebius played a very prominent part. A description of the opening scenes of the Council is given in his Vita Constantini, III.10 sq. After the Emperor had entered in pomp and had taken his seat, a bishop who sat next to him upon his right arose and delivered in his honor the opening oration, to which the Emperor replied in a brief Latin address. There can be no doubt that this bishop was our Eusebius. Sozomen (H. E. I.19) states it directly; and Eusebius, although he does not name the speaker, yet refers to him, as he had referred to the orator at the dedication of Paulinus' church at Tyre, in such a way as to make it clear that it was himself; and moreover in his Vita Constantini, I.1, he mentions the fact that he had in the midst of an assembly of the servants of God addressed an oration to the Emperor on the occasion of the latter's vicennalia, i.e. in 325 a.d. On the other hand, however, Theodoret (H. E. I.7) states that this opening oration was delivered by Eustathius, bishop of Antioch; while Theodore of Mopsuestia and Philostorgius (according to Nicetas Choniates, Thes. de orthod. fid. V.7) assign it to Alexander of Alexandria. As Lightfoot suggests, it is possible to explain the discrepancy in the reports by supposing that Eustathius and Alexander, the two great patriarchs, first addressed a few words to the Emperor and that then Eusebius delivered the regular oration. This supposition is not at all unlikely, for it would be quite proper for the two highest ecclesiastics present to welcome the Emperor formally in behalf of the assembled prelates, before the regular oration was delivered by Eusebius. At the same time, the supposition that one or the other of the two great patriarchs must have delivered the opening address was such a natural one that it may have been adopted by Theodoret and the other writers referred to without any historical basis. It is in any case certain that the regular oration was delivered by Eusebius himself (see the convincing arguments adduced by Stroth, p. xxvii. sq.). This oration is no longer extant, but an idea of its character may be formed from the address delivered by Eusebius at the Emperor's tricennalia (which is still extant under the title De laudibus Constantini; see below, p.43) and from the general tone of his Life of Constantine. It was avowedly a panegyric, and undoubtedly as fulsome as it was possible to make it, and his powers in that direction were by no means slight.

That Eusebius, instead of the bishop of some more prominent church, should have been selected to deliver the opening address, may have been in part owing to his recognized standing as the most learned man and the most famous writer in the Church, in part to the fact that he was not as pronounced a partisan as some of his distinguished brethren; for instance, Alexander of Alexandria, and Eusebius of Nicomedia; and finally in some measure to his intimate relations with the Emperor. How and when his intimacy with the latter grew up we do not know. As already remarked, he seems to have become personally acquainted with him many years before, when Constantine passed through Cæsarea in the train of Diocletian, and it may be that a mutual friendship, which was so marked in later years, began at that time. However that may be, Eusebius seems to have possessed special advantages of one kind or another, enabling him to come into personal contact with official circles, and once introduced to imperial notice, his wide learning, sound common sense, genial temper and broad charity would insure him the friendship of the Emperor himself, or of any other worthy officer of state. We have no record of an intimacy between Constantine and Eusebius before the Council of Nicæa, but many clear intimations of it after that time. In fact, it is evident that during the last decade at least of the Emperor's life, few, if any, bishops stood higher in his esteem or enjoyed a larger measure of his confidence. Compare for instance the records of their conversations (contained in the Vita Constantini, I.28 and II.9), of their correspondence (ib. II.46, III.61, IV.35 and 36), and the words of Constantine himself (ib. III.60). The marked attention paid by him to the speeches delivered by Eusebius in his presence (ib. IV.33 and 46) is also to be noticed. Eusebius' intimacy with the imperial family is shown likewise in the tone of the letter which he wrote to Constantia, the sister of Constantine and wife of Licinius, in regard to a likeness of Christ which she had asked him to send her. The frankness and freedom with which he remonstrates with her for what he considers mistaken zeal on her part, reveal a degree of familiarity which could have come only from long and cordial relations between himself and his royal correspondent. Whatever other reasons therefore may have combined to indicate Eusebius as the most fitting person to deliver the oration in honor of the Emperor at the Council of Nicæa, there can be little doubt that Constantine's personal friendship for him had much to do with his selection. The action of the Council on the subject of Arianism, and Eusebius' conduct in the matter, have already been discussed. Of the bishops assembled at the Council, not far from three hundred in number (the reports of eye-witnesses vary from two hundred and fifty to three hundred and eighteen), all but two signed the Nicene creed as adopted by the Council. These two, both of them Egyptians, were banished with Arius to Illyria, while Eusebius of Nicomedia, and Theognis of Nicæa, who subscribed the creed itself but refused to assent to its anathemas, were also banished for a time, but soon yielded, and were restored to their churches.

Into the other purposes for which the Nicene Council was called, -- the settlement of the dispute respecting the time of observing Easter and the healing of the Meletian schism, -- it is not necessary to enter here. We have no record of the part which Eusebius took in these transactions. Lightfoot has abundantly shown (p.313 sq.) that the common supposition that Eusebius was the author of the paschal cycle of nineteen years is false, and that there is no reason to suppose that he had anything particular to do with the decision of the paschal question at this Council.