the Incarnation of Our Lord against the heretical ideas of Nestorius, who had gained a substantial following with his beliefs that denied that Jesus was both fully man and fully God.
As we have discovered while reading Belloc's The Great Heresies, we have been realizing that attacking the mystery of Our Lord's Incarnation is a generally accepted principle among heresiarchs who attack the teachings of the Church. That God became a Man is mind-blowing when you think about it. If it isn't, maybe you haven't spent enough time thinking this through.
Thankfully, Cyril thought it through and wasn't about to let Nestorius have his way. The result of these controversies was the Council of Ephesus, held in the summer of the year 431. You can read more about this important meeting at the Catholic Encyclopedia. Below is an example of Cyril's rhetorical ability as he explains why Our Lord must have been both fully human and fully God.
Surely it is quite obvious and unmistakable that the Only-begotten became like us, became, that is, a complete man, that he might free our earthly body from the alien corruptions which had been brought into it. He descended to become identical with us, in respect of the conditions of life, accommodating himself through the unity of Word and flesh: he made the human soul his own, thus making it victorious over sin, coloring it, as it were, with the dye of his steadfastness and immutability of his own nature. By becoming the flesh of the Word, who gives life to all things, this flesh triumphs over the power of death and destruction. He is, so to speak, the root and the first fruits of those who are restored in the Spirit to newness of life, to immortality of the body, to certainty and security of divinity, so that he may transmit this condition to the whole of humanity by participation, and as an act of grace.
Cyril also was clear in his argument that the Blessed Mother is the Theotokos or Mother of God as he so clearly and reasonably argues below. Note the high regard he has for Athanatius, who had successfully fought against a similar heretical threat presented by the followers of Arius.
That anyone could doubt the right of the holy Virgin to be called the mother of God fills me with astonishment. Surely she must be the Mother of God if our Lord Jesus Christ is God, and she gave birth to him! Our Lord’s disciples may not have used those exact words, but they delivered to us the belief those words enshrine, and this has also been taught us by the holy fathers.
In the third book of his work on the holy and consubstantial Trinity, our father Athanasius, of glorious memory, several times refers to the holy Virgin as “Mother of God.” I cannot resist quoting his own words: “As I have often told you, the distinctive mark of holy Scripture is that it was written to make a twofold declaration concerning our Savior; namely, that he is and has always been God, since he is the Word, Radiance and Wisdom of the Father; and that for our sake in these latter days he took flesh from the Virgin Mary, Mother of God, and became man.”
Again further on he says: “There have been many holy men, free from all sin. Jeremiah was sanctified in his mother’s womb, and John while still in the womb leaped for joy at the voice of Mary, the Mother of God.” Athanasius is a man we can trust, one who deserves our complete confidence, for he taught nothing contrary to the sacred books.
The divinely inspired Scriptures affirm that the Word of God was made flesh, that is to say, he was united to a human body endowed with a rational soul. He undertook to help the descendants of Abraham, fashioning a body for himself from a woman and sharing our flesh and blood, to enable us to see in him not only God, but also, by reason of this union, a man like ourselves.
It is held, therefore, that there are in Emmanuel two entities, divinity and humanity. Yet our Lord Jesus Christ is nonetheless one, the one true Son, both God and man; not a deified man on the same footing as those who share the divine nature by grace, but true God who for our sake appeared in human form. We are assured of this by Saint Paul’s declaration: When the fullness of time came, God sent his Son, born of a woman, born under the law, to redeem those who were under the law and to enable us to be adopted as sons.
As Venerable Cardinal John Henry Newman would say of him,
Cyril was a clear-headed, constructive theologian. He saw what Theodoret did not see. He was not content with anathematizing Nestorius; he laid down a positive view of the Incarnation, which the Universal Church accepted and holds to this day as the very truth of Revelation. It is this insight into, and grasp of the Adorable Mystery, which constitutes his claim to take his seat among the Doctors of Holy Church. And he traced the evil, which he denounced, higher up, and beyond the person and the age of Nestorius. He fixed the blame upon Theodore of the foregoing generation, "the great commentator," the luminary and pride of the Antiochene school, the master of Theodoret; and he was right, for the exegetical principles of that school, as developed by Theodore, became little less than a system of rationalism.
St. Cyril of Alexandria, pray for us.