Cyril of Alexandria was a prominent Christian theologian and the Patriarch of Alexandria from 412 to 444 AD. He is best known for his role in the early Christian Church and his contributions to the development of Christian doctrine, particularly regarding the nature of Christ. Cyril was a key figure in the Council of Ephesus in 431 AD, where he defended the title of Theotokos for the Virgin Mary, affirming her role as the Mother of God. Cyril's writings include numerous letters and theological treatises that addressed various doctrinal disputes of his time. He was involved in conflicts with other church leaders, notably Nestorius, whose teachings he opposed. Cyril's legacy is significant in the history of Christianity, as he helped shape the understanding of Christ's dual nature as both divine and human.