St. Cyril of Alexandria was probably born between 370 and 380 in Alexandria in Egypt. He, along with his uncle, Patriarch Theophilus of Alexandria, was involved in a dispute between the churches in Egypt and Greece. He may have been a monk before becoming a bishop.
In 412, Cyril succeeded his uncle Theophilus as the head of the Egyptian Church. In 418, the two Eastern churches came back together. But in 428, the churches broke from each other again when the Patriarch of Constantinople refused to refer to Mary as “Mother of God,” instead only calling her “Mother of Christ.”
Cyril continued to uphold the Church’s teachings on Mary, by writing letters to the Nestorius, the Patriarch of Constantinople, and finally by summoning the ecumenical council of 431.
Cyril’s defense of Christ as a single eternally divine person, who became man, was lauded and led to Nestorius’ condemnation.
He died on June 27, 444, after being a bishop for almost 32 years. St. Cyril was declared a Doctor of the Church in 1883.