Saint Cyril
Saint Cyril was the successor and nephew of Theophilus, Pope of Alexandria. As a youth, he entered the monastary of Saint Macarius where he learned the wisdom of the desert monks. Following this, he returned to Alexandria where he was ordained as a priest and rose through the hierarchy until he was finally made Pope of the Egyptian Church. From then on, he began to combat heresy and apostasy, helping to put an end to the Nestorian Heresy and even refuted the Emperor Julian when the latter tried one last time to remove Christianity from the Roman Empire. As Saint Athanasius had fought against Arius, so Cyril now fought against Nestorius. He wrote a letter to Nestorius explaining why his idea of two individual beings in the one person of Christ was heretical, and sent copies to the other Popes of Rome, Antioch, Jerusalem, and Aleppo, as well as to the imperial court. Though Nestorius answered only with contempt, the other letters received favorable replies praising his efforts and offering their support. More letters were written, but Nestorius became more and more stubborn, even trying to antagonize Emperor Theodosius against Cyril.
This controversy eventually became so great that the calling of a synod of bishops seemed the only solution. Theodosius called the First Council of Ephesus to address the heresy, which was held in 431. The bishops present elected Cyril to preside over the council. Though Nestorius was nearby in Ephesus, he refused to appear before the council. Nestorius' beliefs were deemed heretical and a sentence of excommunication was passed by the two hundred bishops in attendance. Six days later, John, Pope of Antioch, arrived with his bishops who had not been able to reach Ephesus in time for the Council. Though they did not believe as Nestorius did, they sided with him against the Council's bishops and deemed him innocent. They met by themselves and issued an accusation of heresy towards Saint Cyril! The Emperor ordered both Saint Cyril and Nestorius confined and the verdicts of both the Council and the Antiochene bishops void. Three legates were dispatched from Rome and when they arrived, they condemned Nestorius and approved of Saint Cyril's decision. The Emperor vindicated Saint Cyril with honor and ordered the exile of Nestorius. The Antiochene bishops contined a minor schism for a time, but made peace with Cyril in 433, agreeing finally with the decision of the Council.
In 1882 Saint Cyril was declared a Doctor of the Church, both for his work defending the faith in the Council of Ephesus as well as for the liturgy that bears his name. According to tradition, the liturgy had been passed down orally beginning with Saint Mark himself, but Saint Cyril completed it and wrote it all down so it would not be forgotten. It is chanted by Christians all over the world during Lent.