"St Cyril was... from Alexandria, born about the year 376, the nephew of Theophilus, Patriarch of Alexandria, who also instructed the Saint in his youth. Having first spent much time with the monks of Nitria, he later became the successor to his uncle's throne in 412. In 429, when Cyril heard tidings of the teachings of the new Patriarch of Constantinople, Nestorius, he began attempting through private letters to bring Nestorius to renounce his heretical teachings about the Incarnation; and when the heresiarch did not repent, Saint Cyril, together with Pope Celestine of Rome, led the Orthodox opposition to his error. Saint Cyril presided over the Third Ecumenical Council of the 200 holy Fathers in the year 431, who gathered in Ephesus under Saint Theodosius the Younger. At this Council, by his most wise words he put to shame and convicted the impious doctrine of Nestorius, who, although he was in town, refused to appear before Cyril. Saint Cyril, besides overthrowing the error of Nestorius, has left to the Church full commentaries on the Gospels of Luke and John. Having shepherded the Church of Christ for thirty-two years, he reposed in 444." (Great Horologion)
  Today we commemorate St Cyril's repose. He is also commemorated on January 18, the date of his restoration to his see in Alexandria after he had been driven out by Nestorians.