"I have found a certain book compiled by someone which contains a large collection of homilies [by Nestorius], that is orderly and systematically arranged for easy use by the reader. And if anything has been said by its author which would come to nothing because it passed into forgetfulness, I would have considered it a duty to hold my peace and to counsel others to do the same, so that things which are unseemly and recklessly said would not become known to others, including our posterity. However, since a multitude of blasphemies have been heaped into this book and a great accusation has been made, howling against the doctrines of the truth, it became necessary that we should strip for combat and fight on behalf of those who have read it, so that they might not be harmed by it, but might rather know how bravely to repulse the damage caused by what is wrong in it.

"St. John was called a son of thunder by Christ the Savior of us all, and with good reason because he said something that is extraordinary and deep, which he sounded forth over all things under heaven and thundered out over the earth. John makes fully known the deep and mighty mystery of the incarnation of the Only begotten. For he said, 'In the beginning was the Word, and the Word was with God, and the Word was God. All things were made through him, and without him was not any thing made that was made' ( Jn 1:1, 3). He pierced through into the mystery and declared that the Only begotten is God and ineffably begotten of God by nature, that He is the maker and creator of all things. Then, and only then at the right time, does He begin the wisest plan that for our sakes and on our behalf was worked. John says, 'And the Word became flesh and dwelt among us, and we have seen his glory, glory as of the only Son from the Father, full of grace and truth' ( Jn 1:14). He said that the Word was flesh, showing the force of the true union, that is, understood as one 'of persons' [according to hypostasis] and by saying that He dwelt among us, he does not allow us to conceive that the Word which is out of God by nature passed into flesh, which is earthly. For someone who is inaccurate about what the divine nature is (which is greater than anything that is created), I might suppose could have deemed that that (divine) nature became the recipient of change and could become, regardless of its own essentially accruing goods, change into something other than what it is and be brought down to the level of the creation, subjected in inappropriate ways to changes and variations. However, this is utterly impossible, for the nature of God is established and has continued unchanged in what it is. John testified saying that 'the Word dwelt among us,' by being made flesh. Here St. John both skillfully explained the wisdom of the divine plan and well defended that the nature of the Word should not be accused by any of having become flesh by change or by turning aside." 

Cyril of Alexandria, Five Tomes Against Nestorius, 1.1