Abstract: Leo the Great was the beneficiary of the consolidation over the course of the preceding century of the power of the papacy in Rome and the Christianization of the city. That process reveals the impact of Ambrose of Milan on the Roman Church and its policy towards the ascetic movement. Leo was heavily influenced by Ambrose and by Augustine of Hippo. His first encounters with theological debate were the Pelagian and Nestorian controversies, where he engaged Cassian as an advisor. He took an admiring though limited view of Cyril of Alexandria but misunderstood the weaknesses in Nestorius'thought. As pope, he preached a civic Christianity, accessible to all citizens, baptizing the virtues of the classical and civic past. His sermons are now dated and reveal the evolution of his thought as he worked out a soteriology that gave full value to both the divinity and humanity of Christ, especially in reaction to Manichaeism. In the crisis that led to Chalcedon, his earlier misunderstanding of Nestorius affected the content of his Tome, which was atypical of the Christology and soteriology he had developed in his earlier preaching. Its emphasis on the distinction of the two natures was an uncharacteristic attempt to respond to both Eutyches and Nestorius, as he understood them. In the light of Chalcedon, he produced a revised statement of Christology, the Letter to the Palestinian monks, which is both more accomplished and better aligned with his characteristic thought.

Keywords: Roman Church, Christianization, Ambrose of Milan, Augustine of Hippo, Pelagian controversy, Nestorian controversy, Cassian, Cyril of Alexandria, Ascetic movement, Manichaeism