CYRIL OF ALEXANDRIA (c. 375–444), church father, theologian, and saint. Cyril succeeded his uncle Theophilus as bishop of Alexandria in 412. His aggressive nature involved him in a series of polemics against heretics. His rhetorical skills were sometimes stronger than his theological judgment, and he was often forgetful of evangelical moderation. In the early days of his studies in the humanities and in religion, he had not been trained to distinguish between the authentic treatises of Athanasius, his most admired predecessor, and those by Apollinarius, listed under Athanasius's name in the episcopal library of Alexandria. Thus he mistakenly urged a form of Christology best expressed by Apollinarius's phrase, which he believed to be Athanasian: "the unique incarnate nature of God the Logos."

Cyril's most famous controversy was with Nestorius, his colleague in the imperial metropolis of Constantinople. A monk from Antioch, made bishop of Constantinople by...