Theodosius I or Theodosius the Great, 346?–395, Roman emperor of the East (379–95) and emperor of the West (394–95), son of Theodosius, the general of Valentinian I. He became (375) military governor of Moesia, but following the execution (376) of his father he retired to Spain. He remained there until Emperor Gratian chose him to rule the East after the defeat and death (378) of Valens in the battle of Adrianople. Theodosius, whom Gratian made co-augustus in 379, took up arms against the Visigoths, who were plundering the Balkan Peninsula. By 381 he had achieved an advantageous peace, permitting the Ostrogoths to settle in Pannonia and the Visigoths in N Thrace. In return he secured their services as soldiers, and soon Gothic influence predominated in the army. In 383, Gratian was murdered; Theodosius was forced to recognize the usurper, Maximus, as emperor in the West outside Italy, where Gratian's brother and legal successor, Valentinian II, held authority. When Maximus seized Italy, Theodosius attacked him, put him to death (388), and restored Valentinian. But Valentinian's Frankish general, Arbogast, assumed the power in Gaul, and in 392, Valentinian, who had sought to recover Gaul, was strangled, perhaps on the order of Arbogast, who installed the puppet emperor Eugenius. Theodosius again went to Italy. In 394 he met a large army commanded by Arbogast and Eugenius and consisting mostly of pagan barbarians. Defeated on the first day of battle, he refused to retreat, and on the following day, with the battle cry
"Where is the God of Theodosius,"
won a resounding victory. Eugenius and Arbogast were slain. Having previously named his son Arcadius as his coemperor in the East, he now proclaimed his younger son, Honorius, as his coemperor in the West. Theodosius died the following year, and the Roman Empire remained divided into West and East. The reign of Theodosius is most notable for its prominence in the history of the Christian Church. Baptized in 380, Theodosius soon afterward issued an edict condemning Arianism and making belief in the Trinity the test of orthodoxy; subsequent edicts practically extinguished Arianism and paganism within the empire. Under his direction the First Council of Constantinople (see Constantinople, First Council of) was convened. The most eminent church figure of his reign was Saint Ambrose, bishop of Milan. When Theodosius ordered a massacre in Salonica to punish the citizens for a rebellion against the garrison, he had to humble himself in the cathedral of Milan before Ambrose lifted his excommunication.
See study by N. Q. King (1960).