Letters And Sermons Of Pope St. Leo I (The Great)Leo's pontificate, next to that of St. Gregory I, is the most significant and important in Christian antiquity. At a time when the Church was experiencing the greatest obstacles to her progress in consequence of the hastening disintegration of the Western Empire, while the Orient was profoundly agitated over dogmatic controversies, this great pope, with far-seeing sagacity and powerful hand, guided the destiny of the Roman and Universal Church. According to the "Liber Pontificalis" (ed. Mommsen, I, 101 sqq., ed. Duchesne, I, 238 sqq.), Leo was a native of Tuscany and his father's name was Quintianus. Our earliest certain historical information about Leo reveals him a deacon of the Roman Church under Pope Celestine I (422-32). Even during this period he was known outside of Rome, and had some relations with Gaul, since Cassianus in 430 or 431 wrote at Leo's suggestion his work "De Incarnatione Domini contra Nestorium" (Migne, P.L.,