an important 4th-century Eastern church bishop who was one of the key proponents of Arianism (the doctrine that Jesus Christ is not of the same substance as God) and who eventually became the leader of an Arian group called the Eusebians.

Eusebius may have met Arius, the Alexandrian priest and originator of Arianism, in Antioch as a fellow student under the theologian and martyr St. Lucian. Eusebius was, successively, bishop of Berytus and, about 318, bishop of Nicomedia. In August 323 Arius wrote Eusebius for aid when his teachings were being investigated by Bishop Alexander. In support of Arius' cause, Eusebius appealed to other bishops. When Arius was condemned in a synod at Alexandria (September 323), Eusebius sheltered him and sponsored a synod (October 323) at Bithynia, which nullified Arius' excommunication.

Eusebius refused to recognize Christ as being “of the same substance” (homoousion) with the Father. Hence, at the first ecumenical Council of Nicaea (Nicaea, Council of), in 325, he led the opposition against the Homoousians (homoousios). When the council finally accepted their clause, Eusebius signed the creed. He refused, however, to sign the anathema condemning the Arians because he doubted “whether Arius really held what the anathema imputed to him.” Shortly after the council he renewed his alliance with Arius, and the Roman emperor Constantine I the Great exiled him to Gaul, where he remained until he presented a confession of faith in 328.

Through his friendship with the emperor's sister, Constantia, he was probably responsible for much of the powerful Arian reaction of the emperor's last years. His unrelenting harassment of the leaders of the Homoousians helped lead Constantine to depose and exile Bishop St. Athanasius the Great of Alexandria at a synod in Tyre in 335 and to reinstate Arius at a synod in Jerusalem in 335. Eusebius was also favoured by Constantine's son and successor, the pro-Arian Constantius II, and was made bishop of Constantinople in 339. He presided over a synod in Antioch in 341—where a creed omitting the homoousion clause was adopted—and he probably died soon afterward.