Theodoret, Church historian, born at Antioch; as bishop of the Syrian city, Cyrus, gave himself to the conversion of the Marcionites; a leader of the Antioch school of theology, he took an active part in the Nestorian and Eutychian controversies, and was deposed by the so-called robber-council of Ephesus, but was reinstated by the Council of Chalcedon in 451 (about 390-457).