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 controversies of his time, including debates with Nestorius, the Patriarch of Constantinople. His work laid the foundation for the Christological debates that would shape Christian theology in the following centuries. Cyril is recognized as a Doctor of the Church and is venerated as a saint in both the Eastern Orthodox and Roman Catholic traditions.