Pope St. Cyril of Alexandria (ca. 378 - ca. 444) was the Pope of Alexandria when the city was at its height in influence and power within the Roman Empire. He is called "The Pillar of Faith" by his supporters, as Pope St. Cyril was Orthodox in faith. Cyril wrote extensively and was a leading protagonist in the Christological controversies of the 4th and 5th centuries. He was a central figure in the First Council of Ephesus in 431 which led to the deposition of Nestorius as Archbishop of Constantinople. Cyril is among the patristic fathers, and the Doctors of the Church, and his reputation within the Christian world has led to his acquiring the title "Seal of all the Fathers." His feast day is celebrated on June 9 and, with St. Athanasius of Alexandria, on January 18.

Controversy
Cyril regarded the embodiment of God in the person of Jesus Christ to be so mystically powerful that it spread out from the body of the God-man into the rest of the race, to reconstitute human nature into a graced and deified condition of the saints, one that promised immortality and transfiguration to believers. Nestorius, on the other hand, saw the incarnation as primarily a moral and ethical example to the faithful, to follow in the footsteps of Jesus. Cyril's constant stress was on the simple idea that it was God who walked the streets of Nazareth (hence Mary was Theotokos or Mother of God), and God who had appeared in a transfigured humanity. Nestorius spoke of the distinct 'Jesus the Man' and 'the divine Logos' in ways that Cyril thought were too dichotomous, widening the ontological gap between man and God in a way that would annihilate the person of Christ.

Theology
As noted above, Cyril was a scholarly archbishop and a prolific writer. In the early years of his active life in the Church he wrote several exegeses. Among these were: Commentaries on the Old Testament [1], Thesaurus, Discourse Against Arians, Commentary on St. John's Gospel [2], and Dialogues on the Trinity. In 429 as the Christological controversies increased, his output of writings was that which his opponents could not match. His writings and his theology have remained central to tradition of the Fathers and to all Orthodox to this day.

Sources

Life and Writings of Cyril of Alexandria as relates to the Christological Controversy
Early Church Fathers Includes text written by Cyril of Alexandria