St. Cyril of Alexandria
Cyril of Alexandria was the Patriarch of Alexandria from 412 to 444. Alexandria was one of the 5 ancient Apostolic Sees in Christianity. It was also one of the great ancient centers of learning, both secular, think of the great library located there, and ecclesial. As one of the apostolic patriarchs Cyril had tremendous influence over Church theology, teaching and discipline.
Cyril was patriarch during the Nestorian conflict. He was the leading bishop at the Council of Ephesus in 431 AD and he fought quite hard for Mary to be called Theotokos, God-bearer, not as his opponents called her Christ-bearer, Christotokos.
Because of his great defense of Christology and Mariology he is one of the doctors of the Church.
His feast day is celebrated June 27th.