An opponent of Nestorius, who held in error that Christ was two persons, one human, one divine, Cyril was an important figure in the Church of the first half of the fifth century. He was made Bishop of Alexandria, which at that time was one of the most important seats in the Church and was a renowned centre of learning, in AD 412.
Cyril was a prolific writer and teacher, many of his writings have come down to us intact. He was a disciple of Athanasius, one of the greatest thinkers in the Church. At the Council of Ephesus in AD431 he vigourously fought Nestorius and Arianism, which claimed Jesus was not fully God, and he also promoted Our Lady as "Theotokos," meaning literally "God-bearer" but which has come down to us a "Mother of God." In a historical time when Dogmas were being defined, particularly the person of Jesus, Cyril was orthodox and of sound doctrine. Some people hold that at Ephesus he had his own agenda but this is only historical word play. The significance of Cyril lay in his ability to fight heresy with truth and his zeal for the work of God.
The writings the Church has chosen for this feast are 1 Timothy 4:1-5 in which Paul tells Timothy to refute falsehood at all cost, and Matthew 5: 13-19, in which Jesus calls on his disciples to be the light of the world and says that he came to fulfill the law not to do away with it. Both these writings admirable apply to Cyril as a theologian and bishop.