LEO I. was only a deacon when be was chosen by the clergy and people of Rome to be their bishop, after the death of Sixtus III., 440, under the reign of Valentiuianus HI., emperor of the West, and Theodosius IL, emperor of the East. He was a man of learning, aud well acquainted with the world and with state affaira, having been employed on several missious by the imperial court. In his youth he had been acquainted with St. Augustine, and had profited by his instruction and example. Soon after his exaltation he had a contro versy with Hilarius, bishop of Arelate (Arles) in Gaul, who had deposed Celidonius, bishop of Yesontio (Besancon), because he had married a widow, which was forbidden by the canons. Celidonius however appealed to Leo, who reinstated him in his see. Hilarius was summoned to Rome upon several charges brought against him by other bishops of Gaul, to whom his severity was obnoxious; aud Leo obtained a rescript from the emperor Valcntiuiau suspending Hilarius from his episcopal office. This suspension however does not appear to have been lasting, although the fact has been taken hold of by controversial writers as a stretch of jurisdiction in the see of Rome. Quesuel published a dissertation upon this controversy in his edition of the works of Leo, Paris, 1675. Leo also induced the emperor to issue, in the year 445, several laws against the Manichmans and other heretics, depriving them of the right of citizenship and of inheritance, and excluding them from the military service. He assembled a council at Rome in 449, in which he annulled the acts of the council of Ephesus, which had absolved Eutyches. (EnTrems.] Soon afterwards the (Ecumenic council of Chalcedon, 451, in which Leo's legates presided, condemned the doctrine of Eutyches, and defined the doctrine concerning the person of Christ. By a canon of
this council, which was cecummle, or universal, both for the East and West, the Bishop of Coostant-ineple was declared to be next in place, though equal in dignity, to the Bishop of Rome, and the limits of their respective juriedictiom were determined, the patriarchates of Antioch and Alexandria beingplaced under that of Constantinople ; which canon passed the assembly, notwithstanding the opposition of the Roman legates. The story of Leo stopping Attila on his march, and persuading him to spare the city of Rome, ie an embellishment ; but it appears that Lou was really sent by Valeutiniau on a mission to Attila, who was then devastating Lombardy, and that Attila con sented to a truce with Valentiuien, after which he recrossed the Alps. Some years after, Leo did prevail upon Gametic, who had landed at the mouth of the Tiber 455, to spare at lent the lives and the buildings in Rome, and not to allow his Vandals to sot fire to that city or slaughter the inhabitants. Genserie was satisfied with the plunder of Rome, and returned to Africa. Leo died in 461, and was suc ceeded by Marius I. his writings, especially his Sermons and his Epistles. are useful for the history of the times. Quesnel has giveu a full account of his life, as well as Maimbourg, 'Histoire du Pontificat de St. lAen le Grand.' Father Cacciari published en edition of Leo's works, 3 vols. folio, Rome, 1751.55, in which he has charged Quesnel's edition with great incorrectness. Leo's Sermons have been translated into French by the Abb6 de Bellegarde, Paris, 1701. The Roman church numbers him *Along its saints, and gives him also the epithet of Msguus, or "St. Leo the Great."