Of a good family and a moderate Arian, as bishop of Beroea Thrace he was part of an eastern delegation that presented the Ekthesis makrostichos to Constans at Milan in 345. In 351 he participated at the Council of Sirmium which deposed Photinus; his presence in 359 at Rimini is uncertain. In 356–357 Pope Liberius was exiled to Beroea, near him; Demophilus must have had a hand in the pope’s decision to comply and subscribe the condemnation of Athanasius. In 370 Valens promoted his election to bishop of Constantinople to succeed the deceased Eudoxius, who had been bishop of the capital for 11 years.
Demophilus, ordained by Arian bishops, supported the emperor’s pro-Arian activity in the East, but was also very hostile to radical Arians. In 380 he refused Theodosius’s order to subscribe the Nicene Creed and was forced to abandon his see and return to his native Thrace Socrates, HE 5,7; Philostorgius, HE 9,10 and 14. In 383 the emperor asked of him a profession of faith as leader of the moderate Arians; it has not reached us Socrates, HE 5,10: PG 67, 688- 693; Sozomen, HE 7,12: PG 67, 1445. Philostorgius HE 9,14 has him professing a monophysite Christology ante litteram. Upon his death Marinus was elected as the Arian bishop, but Dorotheus, then at Antioch, was later selected Sozomen, HE 7,14. Hfl-Lecl 1,918-928; DHGE 14, 212-215; H.C. Brennecke, Studien zur Geschichte der Homöer, Tübingen 1988; A. Martin, Athanase d’Alexandrie et l’Église d’Égypte au IVe siècle 328–373, Rome 1996, passim.