Patr. of Alexandria from 412. The most important of the many conflicts in which he engaged arose out of the support given by Nestorius, Patr. of Constantinople, to a chaplain who objected to the application of the word Theotokos (the ‘one who gave birth to God’) to the BVM on the ground that she was the mother of only the humanity of Christ. Cyril defended the contested word in his Paschal letter for 429. He then persuaded Pope Celestine I to summon a synod at Rome in 430 and condemn Nestorius. Cyril, who had been delegated to act for Celestine, had the condemnation repeated in his own synod at Alexandria and sent notice of both decrees to Nestorius with a covering letter, appending for his acceptance twelve anathemas. At the Council of Ephesus (431) he had Nestorius deposed before the Antiochene bishops arrived. They then held a separate council and deposed Cyril, but in 433 he reached agreement with the moderate Antiochenes.
The most brilliant representative of the Alexandrian theological tradition, Cyril put into systematic form the classical Greek doctrines of the Trinity and of the Person of Christ. His writings are marked by precision in exposition, accuracy in thought, and skill in reasoning, though they lack elegance. They include letters, exegetical works, treatises on dogmatic theology, and sermons. Feast day in the E., 9 June; in the W., 27 June (formerly, 9 Feb.).