Saints are not born with halos around their heads. Cyril, recognized as a great teacher of the Church, began his career as archbishop of Alexandria, Egypt, with impulsive, often violent, actions. He pillaged and closed the churches of the Novatian heretics, participated in the deposing of St. John Chrysostom and confiscated Jewish property, expelling the Jews from Alexandria in retaliation for their attacks on Christians.

Cyril’s importance for theology and Church history lies in his championing the cause of orthodoxy against the heresy of Nestorius.

The controversy centered around the two natures in Christ. Nestorius would not agree to the title “God-bearer” for Mary. He preferred “Christ-bearer,” saying there are two distinct persons in Christ (divine and human) joined only by a moral union. He said Mary was not the mother of God but only of the man Christ, whose humanity was only a temple of God. Nestorianism implied that the humanity of Christ was...