Early Life and Election
Leo was born about the year 400 in Tuscany, of an aristocratic family, and received an excellent education. He entered the service of the Roman Church and by the early 430s was a deacon of sufficient standing that the monastic writer John Cassian dedicated a theological work to him; he served under Popes Celestine I and Sixtus III. Toward the end of Sixtus III's reign the Emperor Valentinian III entrusted him with a mission to Gaul to settle a quarrel between the general Aetius and the magistrate Albinus.
When Sixtus III died, Leo was unanimously elected Bishop of Rome and consecrated on 29 September 440. He would govern the Roman Church for the next twenty-one years, a period marked both by his defense of doctrine and by the political turbulence of the failing Western Empire.
The Tome and the Council of Chalcedon
Leo's most enduring contribution was doctrinal. In 449 he addressed a dogmatic letter, known as the Tome of Leo, to Flavian, Archbishop of Constantinople, setting out the teaching that in Christ two natures, divine and human, are united in one person without confusion or division. The letter answered the error of Eutyches, who had taught that Christ possessed only a single nature.
At the Fourth Ecumenical Council, held at Chalcedon in 451, the Tome was read before the assembled bishops, who recognized in it the apostolic faith and exclaimed, by tradition, "Peter has spoken through Leo." The council confirmed the Orthodox confession of Christ and condemned the heretics Eutyches and Dioscorus of Alexandria. Leo had actively promoted the gathering of the council, and his letter became one of its foundational dogmatic documents.
Defender of Rome
Leo's authority extended into the civil crises of his age. In 452, as Attila and his Huns advanced upon Italy, Leo went out to meet the king and, according to the contemporary chronicler Prosper of Aquitaine, persuaded him to withdraw without sacking Rome. The synaxarion relates that Attila yielded both for the holiness of Leo and because of a vision of the Apostles Peter and Paul standing behind him, threatening the king with a flaming sword.
Three years later, in 455, when Gaiseric and the Vandals took Rome, Leo again interceded; his intervention is credited with sparing the city from wholesale burning and with preserving its principal basilicas. He labored as well against the heresies of Pelagianism, Manicheism, and Priscillianism, and worked to strengthen the order and discipline of the Western Church.
Writings
A large body of Leo's preaching and correspondence survives — by the usual count, some ninety-six sermons and about a hundred and forty-three letters. The sermons treat the feasts and seasons of the Church year and the duties of the Christian life, while the letters address the doctrinal and disciplinary questions of his time. Of all of them the Tome to Flavian is the most renowned, for the place it holds in the history of Orthodox Christology.
Relics & Shrines
Leo reposed on 10 November 461 and was buried in Old St. Peter's Basilica in Rome, the first pope to be interred there. In 688 Pope Sergius I had his remains translated to a more prominent place in the south transept of the basilica, where they continued to be venerated.
Veneration
Leo is venerated as a saint and confessor in both the Eastern Orthodox and the Western Churches, since he belongs to the undivided Church before the later divisions of Christendom. In the Orthodox East his feast is kept on 18 February, while in the West it is observed on 10 November. In the Roman tradition he was later named a Doctor of the Church.
