EUDOSSIO. — One of the principal leaders of Arianism in the 4th century, bishop of Constantinople from 360 to 370. Born at Arabissus, in Lesser Armenia, he attended in Antioch the lectures of St. Lucian the Martyr (d. 311), and subsequently embraced Arianism. Bishop of Germanica, he took part in the Council of Antioch of 341, known as the Council of the Dedication. He was also present at most of the numerous synods convened by Constantius: at Sardica in 343, among the Eastern bishops; probably at Sirmium in 351; certainly at Milan in 355 and at Sirmium in 357. From that city, on hearing of the death of Leontius, the Arian bishop of Antioch, he set out precipitously to obtain his succession. He succeeded in this by trampling upon the canons, despite the protests of the electing bishops. From that moment he openly declared himself for strict Arianism, or Anomoeanism, represented by Aëtius and Eunomius. But he soon found himself in difficulty, since the emperor Constantius, following the Council of Ancyra of 358, had declared in favor of the semi-Arianism and Homoousianism of Basil of Ancyra. Having run the risk of being exiled and deposed, he temporarily withdrew from Antioch. But he soon gained his revenge alongside Acacius of Caesarea, with whom he contributed to the triumph of the formula of Rimini-Constantinople, that is, of Homoeanism, which became official Arianism. Constantius rewarded him for such doctrinal compliance by transferring him to the see of Constantinople (360), which he occupied until his death (370). If he managed to retain this position, he owed it to the protection of the Arian emperor Valens, whom he himself baptized before the expedition against the Goths (366–67).
E. was an ambitious man without character or firm convictions. The Arian Philostorgius describes him as an amiable man of irreproachable conduct, but timid to the point of cowardice. Of his writings, which, it seems, were numerous, there remain only fragments of doubtful authenticity and a brief profession of faith, taken from a treatise on the Incarnation: Λόγος περί αἰφιώσεως (cf. P. Caspari, Alte und neue Quellen zur Geschichte des Taufsymbols und der Glaubensregel, Cristiania 1879, pp. 170–85).