Cyril became patriarch of Alexandria in 412 and dominated the Christological argument of his century. Against Nestorius he pressed the case that Christ is a single subject — that it is God the Word who is born, hungers, suffers and dies, and that Mary may therefore rightly be called Theotokos, God-bearer.
That position carried at the Council of Ephesus in 431 and has been the mainstream of Christian thought since. Cyril was a formidable and often ruthless controversialist, and his career includes episodes his admirers do not defend. But his writing on the unity of Christ is of the first rank, and he is held in particular honour in the Oriental Orthodox churches, whose Christology descends directly from his.