Leo (1) I., emperor (surnamed the Great, the Thracian, and the Butcher), born c. 400 in the country of the Bessi in Thrace, proclaimed emperor Feb. 7, 457, and crowned by Anatolius, patriarch of Constantinople, being the first Christian sovereign to receive his crown from the hands of a priest. Immediately upon the news of Marcian's death, religious troubles broke out in Alexandria, where the Monophysite party murdered the patriarch Proterius (Proteius), substituting for him Timothy Aelurus. The orthodox bishops of Egypt fled to the emperor to make complaint. Anatolius, bp. of Constantinople, reported their sad case to pope Leo, who energetically seconded their efforts for redress. The emperor, distracted by the demands of pope and patriarch on the one hand, of Aspar and the heretical party on the other, addressed a circular letter to Anatolius and all other metropolitans, commanding them to assemble their provincial councils, and advise him—(1) whether the decrees of the council of Chalcedon should be held binding; (2) as to the ordination of Timothy Aelurus. He also consulted the three most celebrated ascetics of the time, Symeon Stylites, James the Syrian, and Baradatus. We possess in the Codex Encyclius the answers of all the bishops and hermits consulted, a most valuable monument of ecclesiastical antiquity. It was apparently composed by imperial order by some unknown Greek, translated into Latin at the order of the senator Cassiodorus by Epiphanius Scholasticus, and first published in modern times by Laurentius Surius. It is in all collections of the councils, but in full only in Labbe and Coss. Concil. i. 4, pp. 890–980 (cf. Cave, Scriptt. Lit. Hist. i. 495; Tillem. Mém. xv. art. 167). The bishops, in Aug. 458, replied, unanimously upholding the decrees of Chalcedon and rejecting the ordination of Timothy, who, however, maintained his position at Alexandria till 460.
In 468 Leo sent an expedition under the command of Basiliscus, his brother-in-law, against the Arian Vandals of N. Africa, who were bitterly hostile to him on account of his orthodoxy. Aspar and Ardaburius secretly arranged with Basiliscus for its failure, as they feared any diminution of the great Arian power. The emperor, having discovered the conspiracy, put Aspar and Ardaburius to death, and banished Basiliscus a.d. 469. The Gothic guards, in revenge, raised a civil war in Constantinople, under one Ostrys, a friend of Aspar, and attacked the palace, but were defeated. Leo thereupon issued a severe edict against the Arians and forbade them holding meetings or possessing churches.
In another quarter controversy burst forth. Gennadius, patriarch of Constantinople, dying in 471, was succeeded by Acacius, whom Leo admitted a member of the senate, where no ecclesiastic had hitherto sat. Acacius obtained from Leo an edict confirming the 28th canon of Chalcedon, which raised Constantinople to the same ecclesiastical level as Rome. Pope Simplicius resisted the claim, and a bitter controversy ensued, lasting many years and most fruitful in divisions (Milman, Lat. Christ. lib. iii. c. i.).
Leo was very active in church legislation. He made laws in 466 confirming the right of asylum to churches; in 468 forbidding any persons save Christians to act as advocates. In 469 he issued an edict against simoniacal contracts and one of almost puritan strictness upon the observance of Sunday. He forbade judicial proceedings on that day, and even the playing of lyre, harp, or other musical instrument (Chron. Pasch. a.d. 467, where the words of the edict are given). The same year he passed stern laws against paganism and issued a fresh edict in favour of hospitals. In 471 a law was published, apparently elicited by the troubles at Antioch, commanding monks not to leave their monasteries. When Isocasius, a philosopher and magistrate of Antioch, was forced by torture to accept baptism at Constantinople, the emperor seems to have personally superintended the deed (Joan. Malalas, Chronogr. lib. xiv.). Leo died Feb. 3, 474, aged 73, and was succeeded by his grandson Leo II. Evagr. H. E. lib. ii.; Procopii, de Bell. Bandal.; Theoph. Chronogr.
[G.T.S.]