9 To be convinced of this we have but to consider the way in which he discharged his office as teacher of the Catholic Faith. In this field he won for himself a name equal to that of St. Augustine of Hippo and St. Cyril of Alexandria. St. Augustine, as we know, in defending the Faith against the Pelagians, insisted on the absolute necessity of divine grace for right living and the attainment of eternal salvation. St. Cyril, faced with the errors of Nestorius, upheld Christ's Divinity and the fact that the Virgin Mary is truly the Mother of God. These truths lie at the very heart of our Catholic Faith, and St. Leo, who entered into the doctrinal inheritance of both these men of learning, the brightest luminaries of the Eastern and Western Church, was among all his contemporaries by far the most fearless protagonist of them.
Source: Aeterna Dei Sapientia (Vatican.va)