One of the most important (post-NT) theologian in the history of Christianity is almost unknown in the West. St. Cyril of Alexandria died in 444. His thought dominated four ecumenical councils (Ephesus, Chalcedon, II Constantinople, and III Constantinople) and several of his letters are simply affirmed by these councils as orthodox.
He is best known for being, as Patriarch of Alexandria, the opponent of Nestorius, the Patriarch of Constantinople. It was a very serious dispute, since the four major Patriarchs were split : Antioch (whose bishop was John) and Constantinople were Nestorian, Alexandria and Rome (whose bishop was Celestine) were Cyrilline. So serious was the dispute that, to keep the peace, the Emperor Theodoret had to call a council. The outcome was the Council of Ephesus, where Nestorius was deposed as bishop. The Antiochene delegation was delayed to the Council, and Cyril made the serious error of opening the council instead of waiting. In response, John and his supporters formed a counter-council that issued a declaration saying that Memnon (bishop of Ephesus) and Cyril were deposed. Eventually, the counter-council's deposition was considered invalid and John and Cyril were reconciled; Cyril's letter to John upon reconciliation was accepted by the Council of Chalcedon.
Thanks to the Internet you can have easy access to translations of the conciliar decrees and some of Cyril's most important works.
Council of Ephesus
Council of Chalcedon
Cyril of Alexandria
Cyril's writings at Early Church Fathers