From the LCMS website:
Cyril (ca. A.D. 376-444) became archbishop of Alexandria, Egypt, in 412. Throughout his career he defended a number of orthodox doctrines, among them the teaching that Mary, the mother of Jesus, is “rightly called and truly is the Mother of God”–Theotokos, “the God-bearer” (Formula of Concord, VIII, Ep VIII, 12). In 431 the Council of Ephesus affirmed this teaching that the Son of Mary is also true God. The writings of Cyril on the doctrines of the Trinity and the person of Christ reveal him to be one of the most able theologians of his time. Cyril’s Christology influenced subsequent church councils and was a primary source for Lutheran confessional writings.