Letter 39, Formula of union between Cyril and John of Antioch (433):
Concerning the Virgin Mother of God, we thus think and speak; and of the manner of the Incarnation of the Only Begotten Son of God, necessarily, not by way of addition but for the sake of certainty, as we have received from the beginning from the divine Scriptures and from the tradition of the holy fathers, we will speak briefly, adding nothing whatever to the Faith set forth by the holy Fathers in Nice. For, as we said before, it suffices for all knowledge of piety and the refutation of all false doctrine of heretics. But we speak, not presuming on the impossible; but with the confession of our own weakness, excluding those who wish us to cling to those things which transcend human consideration.
We confess, therefore, our Lord Jesus Christ, the Only Begotten Son of God, perfect God, and perfect Man of a reasonable soul and flesh consisting; begotten before the ages of the Father according to his Divinity, and in the last days, for us and for our salvation, of Mary the Virgin according to his humanity, of the same substance with his Father according to his Divinity, and of the same substance with us according to his humanity; for there became a union of two natures. Wherefore we confess one Christ, one Son, one Lord. According to this understanding of this unmixed union, we confess the holy Virgin to be Mother of God; because God the Word was incarnate and became Man, and from this conception he united the temple taken from her with himself. For we know the theologians make some things of the Evangelical and Apostolic teaching about the Lord common as pertaining to the one person, and other things they divide as to the two natures, and attribute the worthy ones to God on account of the Divinity of Christ, and the lowly ones on account of his humanity [to his humanity].
Letter 44 to Eulogius:
Some attack the exposition of faith which those from the East have made and ask, "For what reason did the Bishop of Alexandria endure or even praise those who say that there are two natures?" Those who hold the same teachings as Nestorius say that he thinks the same thing too, snatching to their side those who do not understand precision. But it is necessary to say the following to those who are accusing me, namely, that it is not necessary to flee and avoid everything which heretics say, for they confess many of the things which we confess. For example, when the Arians say that the Father is the creator and Lord of all, does it follow that we avoid such confessions? Thus also is the case of Nestorius even if he says there are two natures signifying the difference of the flesh and the Word of God, for the nature of the Word is one nature and the nature of his flesh is another, but Nestorius does not any longer confess the union as we do.
For we, when asserting their union, confess one Christ, one Son, the one and same Lord, and finally we confess the one incarnate phusis of God.2 It is possible to say something such as this about any ordinary man, for he is of different natures, both of the body, I say, and of the soul. Both reason and speculation know the difference, but when combined then we get one nature of man. Hence knowing the difference of the natures is not cutting the one Christ into two.
Letter 46 to Succensus:
But if this is true, how will we not be granting that the two natures subsist indivisibly after the union? So that if one says "that Christ therefore suffered in his flesh"20 for us, he is saying nothing else except that Christ therefore suffered for us in our nature.
Again this question is no less in opposition to those who say that there is one incarnate nature of the Son, and the proposers, desiring to prove that this formula is rather useless, eagerly strive to prove that two natures always subsisted. But they have ignored the fact that those things which are usually distinguished not just according to speculation, completely and specifically differ from one another in every manner separately into diversity. Let a man like unto us be an example for us again. For we know that there are two natures in him, one the nature of the soul and the other the nature of the body. But when we divide him merely in thought and conceive the difference in subtle speculations or the presentations of thought to the mind, we do not posit the natures one apart from the other, nor indeed do we at all impute to them virtual existence through the division, but we conceive of them as the natures of one man, so that the two no longer are two, but
through them both the one living being is produced.