Cyril of Alexandria  was one of the most important bishops and theologians of the early Church. He became bishop of Alexandria in 412, succeeding his uncle Theophilus, and quickly emerged as a leading defender of Nicene orthodoxy.

Cyril is best known for his conflict with Nestorius, bishop of Constantinople, whose teaching appeared to divide Christ too sharply into a divine person and a human person. Against this, Cyril insisted that Jesus Christ is one person: the eternal Son of God truly made flesh. For this reason, he defended the title Theotokos, “God-bearer,” for Mary, not chiefly to exalt Mary, but to protect the truth of Christ’s incarnation.

His teaching was central at the Council of Ephesus in 431, where Nestorius was condemned. Through his letters, biblical commentaries, and theological writings, Cyril helped shape the Church’s confession that Christ is fully God and fully man, united in one person. He remains one of the great theologians of the Nicene and post-Nicene era. 

“Not in these therefore (I mean the ordinances of the Law) is true salvation, nor yet will any one win hence the thrice-longed for freedom, I mean from sin. But bounding a little above the types, and surveying the beauty of the worship in Spirit and acknowledging the Truth, that is Christ, we are justified through faith in Him, and justified we pass over unto the true liberty, ranked no more among slaves as heretofore, but among the sons of God. And John will testify this, saying of Christ and of them that believe on Him, But as many as received Him, to them He gave power to become children of God.

Profitably then doth our Lord and Christ not suffer them who believe on Him to marvel any more at the shadows of the law (for there is nought in them that profits or that bestows the true freedom) but bids them rather know the Truth; for through this does He say that they shall be entirely freed, according to the mind of the words.

We be Abraham's seed and have never been in bondage to any man, how sayest Thou, Ye shall be made free?

They laugh at the promise of our Saviour, rather they even take it ill, as though they were insulted. For that which has no share at all of bondage, how will it need (he says) of One Who calls us unto freedom, and Who gives us a something over and above what is in us already.

But they know not, though wont to have a conceit of being wise, that their forefather Abraham was of no notable father after the world, nor yet of highest repute among those who are admired in this life, but was ennobled by faith only in God: Abraham believed God, it says, and faith was imputed to him for righteousness and he was called the Friend of God. Thou seest then very clearly the cause of his illustriousness.

For since he was called the friend of God who ruleth over all, he hath become on this account great and famed, and his faith was imputed to him for righteousness, and the righteousness which is of faith hath become to him the cause of freedom towards God. Therefore when he by believing was justified, that is, when he shook off the low birth that is from sin, then did he appear illustrious and of noble birth and free.”

Cyril, Comm. John 5.5, on John 8:32–33, in Pusey, Commentary on John, 1:625–626. [bold added]