Cyril was a successor of St Athanasius in the see of Alexandria, the great champion of the Council of Ephesus, the opponent of Nestorianism, and the defender of Mary's title of Theotokos, or "Mother of God." He succeeded his uncle Theophilus as patriarch of Alexandria in 412 and had a part in the condemnation of St John Chrysostom in 403.
His doctrinal significance stems from his opposition to Nestorianism and the dominant role he played in the Council of Ephesus in 431. Nestorius, who was patriarch of Constantinople, objected to the title of Mother of God, and preferred her to be called Mother of Christ. Cyril wrote to Nestorius trying to correct his error, appealed to the pope in Rome for a clarification of the true doctrine, and opened the council called at Ephesus by Emperor Theodosius II. The council sessions were stormy, but Nestorius was condemned and Mary's title Theotokos became her official title in the Church.
The Church, however, was divided at Ephesus, even though Cyril did much to bring about unity. The problem of the "two natures and one person" in Christ did not see its final clarification until the Council of Chalcedon in 451, but by that time Cyril was dead. He died in 444, venerated by the Christian world, one of the great champions of the Mother of God.
In 1881, he was declared a Doctor of the Church by Pope Leo XIII, and in 1944, on the fifteenth centenary of Cyril's death, Pope Pius XII issued his encyclical Orientalis Ecclesiae, commemorating Cyril's place in the history of the Church.
From Rev. Clifford STEVENS, The One Year Book of Saints, p. 196, Our Sunday Visitor, Inc., 1989.