Leo I was born c.400 AD in Tuscany. By the age of 31, while he was a deacon, he had already established enough influence and credibility that he was able to apply to Cyril of Alexandria that Rome’s patriarchal jurisdiction should take priority in Palestine over that of the claims of the Juvenal of Jerusalem. At the same time, John the Ascetic (who would later become Saint John Cassian), dedicated to him the treatise against Nestorius, the Archbishop of Constantinople, written at his request. Even the emperor of Rome, Theodosius II, respected Leo to the point he was chosen to settle the dispute between the two highest officials in Gaul – Flavius Aetius and Caecina Decius Aginatus Albinus. While Leo was on the mission to Gaul, Pope Sixtus III died and Leo was unanimously elected to succeed him, becoming Pope Leo I the Great on September 29, 440.
Theologically, Leo is most remembered for his issuance of the Tome of Leo – a letter he wrote to the Council of Chalcedon explaining the Papal position in the matters of Christology. His position explained that Jesus Christ had two natures, and was not of, or from, two natures. Therefore, Christ was both divine and human, united. This position put the Papacy at odds with various other factions of Christianity and formed a major schism to Monophysitism (Christ had only a single nature after the union of the divine and human); Miaphysitism (Christ had two joined natures of divine and human); and Dyophysite (Christ had two natures, separate, but existing together).
Leo dedicated himself to the centralization of spiritual authority to Rome and the Papacy, shifting the power away from the local bishops throughout the Roman Empire. Through the important decree by Valentinian III (Western emperor 425-455) on June 6, 455, Leo successfully appealed to the emperor to recognize the primacy of the bishop of Rome, based on the merits of Saint Peter and the First Council of Nicaea. Leo also dedicated his life and teachings based on his personal devotion to Saint Peter, which is why he felt the primacy needed to be with the bishop of Rome – Saint Peter being the connection to Christ through whom Leo communicated.
Although Pope Leo was clearly primarily involved in matters of the Church, he also performed one extremely important task for the emperors Valentinian III and Marcian (Eastern emperor 450-457) – he was involved in the meeting with Attila the Hun during the invasion of Italy and headed to Rome. Attila utterly razed Aquileia in 452 and was headed toward Rome, to claim the sister of Valentinian III, Justa Gratia Honoria, and a dowry of half the Western Empire. Leo, along with two other envoys, (consul in 450) Gennadius Avienus and former urban prefect Memmius Aemilius Trygetius, met with Attila to negotiate. Nothing concrete is known about the meeting other than it happened, but the result was the withdrawal of Attila and his forces. Some speculate Attila was so moved and impressed with Leo that he decided to abandon his plans to sack the city. Leo died on November 10, 461, having served as pope for 21 years. He was sainted and November 10 is celebrated by Roman Catholics as the feast day of Saint Leo.