It was during this visit to Constantinople that another synod was held, at which Eusebius was present, and the result of which was the condemnation and deposition of the bishop Marcellus of Ancyra (see Socrates, I.36; Sozomen, II.33; Eusebius, Contra Marc. II.4). The attitude of our Eusebius toward Marcellus is again significant of his theological tendencies. Marcellus had written a book against Asterius, a prominent Arian, in which, in his zeal for the Nicene orthodoxy, he had laid himself open to the charge of Sabellianism. On this account he was deposed by the Constantinopolitan Synod, and our Eusebius was urged to write a work exposing his errors and defending the action of the Council. As a consequence he composed his two works against Marcellus which will be described later. That Eusebius, if not in the case of Athanasius and possibly not in that of Eustathius, had at least in the present case good ground for the belief that Marcellus was a Sabellian, or Sabellianistic in tendency, is abundantly proved by the citations which he makes from Marcellus' own works; and, moreover, his judgment and that of the Synod was later confirmed even by Athanasius himself. Though not suspecting Marcellus for some time, Athanasius finally became convinced that he had deviated from the path of orthodoxy, and, as Newman has shown (in his introduction to Athanasius' fourth discourse against the Arians, Oxford Library of the Fathers, vol.19, p.503 sq.), directed that discourse against his errors and those of his followers.

The controversy with Marcellus seems to have been the last in which Eusebius was engaged, and it was opposition to the dreaded heresy of Sabellius which moved him here as in all the other cases. It is important to emphasize, however, what is often overlooked, that though Eusebius during these years was so continuously engaged in controversy with one or another of the members of the anti-Arian party, there is no evidence that he ever deviated from the doctrinal position which he took at the Council of Nicæa. After that date it was never Arianism which he consciously supported; it was never the Nicene orthodoxy which he opposed. He supported those members of the old Arian party who had signed the Nicene creed and protested that they accepted its teaching, against those members of the opposite party whom he believed to be drifting toward Sabellianism, or acting tyrannously and unjustly toward their opponents. The anti-Sabellianistic interest influenced him all the time, but his post-Nicene writings contain no evidence that he had fallen back into the Arianizing position which he had held before 325. They reveal, on the contrary, a fair type of orthodoxy, colored only by its decidedly anti-Sabellian emphasis.