St. Cyril, Patriarch of Alexandria, one of the metropolitan sees of the Christian Church in the east, was one of the great defenders of the faith against the heresy of Nestorius who denied the oneness of person in Jesus Christ. At the Council of Ephesus in 431, over which he presided in the Pope's name, and at his instigation, it was defined that Christ, the Son of God, is at the same time God and man, and the Blessed Virgin Mary, His mother, is truly the Mother of God. St. Cyril died in 444. The Church venerates him as one of her great doctors. His commentary on the Gospel of St. John is one of the richest doctrinally of those left us by the Fathers of the Church. The optional feast of St. Cyril of Alexandria is June 27th.
Also see
excerpts of St. Cyril's writings at The Crossroads Initiative.