The points Saint Cyril is making here are: first, the two natures are united in an indivisible union,and second, that after the union, we do not divide the natures from one another, third, we can only theoretically speak about Christ being out of two natures (not in two natures), and fourth, that as we have received from the fathers we should speak of the One Incarnate Nature of The Word.
We speak of two natures which came into union, (but) after the union, inasmuch as that which divides them into two is eradicated, we confess that there is one nature of the Son as being one, Who, however, became man and was incarnate. ... We do not at any time eradicate the difference between the expressions, although we have rejected dividing them out separately ... For, as we are persuaded, the nature of God the Word is one, God the Word who was both incarnate and made man.3
So when the manner of the incarnation is investigated, the human mind observes those things mutually joined in an ineffable union without alteration or intermingling. After they have become one, the mind does not divide them at all, but believes and truly accepts, that there is one God, Son, Christ and Lord who is from both of them. But the wickedness of Nestorius’s belief is different from this. For he pretends to acknowledge that God the Word was incarnate and made man. (But) because he does not know the import of the incarnation, he names “two natures”, separating them from one another.4
Saint Cyril insisted on using the formula One Incarnate Nature of The Word in view of Nestorius using the two nature terminology to defend his opinions. Nestorians accused Saint Cyril that he teaches that the body of Christ is changed into the Godhead. He responded to these objections:
It would be just as foolish an idea to talk of the body being transformed into the nature of Godhead as it would to say the Word was transformed into the nature of flesh. For just as the latter is impossible (for he is unchangeable and unalterable) so too is the former. It is not possible that any creature could be converted into the essence or nature of Godhead, and the flesh is a created thing. We maintain, therefore, that Christ's body is divine in so far as it is the body of God, adorned with unspeakable glory, incorruptible, holy, and life-giving; but none of the holy Fathers has ever thought or said that it was transformed into the nature of Godhead, and we have no intention of doing so either.5
“If Emmanuel is composed out of two natures, but after the union there is One Incarnate Nature of the Word, it then follows that we must assert he suffered in His own nature.” To this saint Cyril responds:
Were it the case that within the conditions of the providential dispensation there was nothing capable of suffering, suffering must occur to the Word’s nature. But if through our calling His nature “incarnate” the whole concept of the incarnate dispensation is brought up, ... how can they jabber about it being necessary for Him to suffer in His nature, when it is His body which incurs the suffering whilst the Word is impassible? Yet we do not dissociate Him from suffering. For just as His body became his own, so all things pertaining to the body except sin are by providential appropriation, predicated of Him.6
Later on he adds:
They do not understand the dispensation but are artfully transferring the suffering to the man on his own and are foolishly engaging in an injurious piety, so that God the Word shall not be acknowledged as Saviour, as the one who gave His blood for our sake ... but this view entirely uproots the incarnate dispensation and substitutes Man-worship for our glorious mystery. They do not perceive that it is He who is “descended from the Jews in the flesh” i.e. “Who is Christ of the seed of Jesse and David” whom blessed Paul called “Lord of Glory” (and) “God blessed forever.”7
“If there is One Incarnate Nature of the Word, it is absolutely necessary to speak of mixture as having occurred inasmuch as the human nature has been reduced and filched (stolen) away.” To this saint Cyril responds:
The ineffable and unspeakable union has revealed to us the Son’s Single, but as I have said,Incarnate nature. For singleness is not predicated truly only of beings simple by nature, but also of beings brought together in composition, such as Man, who is compounded of body and soul. For these elements are heterogenous and are in nature mutually dissimilar; but united they make the single nature of Man ... Those then who assert that “If there is One Incarnate Nature of the Word, it is absolutely necessary to speak of mixture as having occurred inasmuch as the human nature has been reduced and filched away”, are talking rubbish. For it is neither “reduced” nor, as they say, “filched away”. For it suffices for a complete indication of the fact that He became man that we should say that He was Incarnate. Were this not avowed by us, there would be room for their cavilling. But since “Incarnate” must be added to “One Nature”’where is there any sort of reduction or filching away?8
“If anyone says that Christ suffered for our sakes in the flesh, he is affirming none other than this: that Christ suffered for our sake in our nature (the human nature)”. To this Saint Cyril replies:
Let Man be our illustration again. For we think of him as two natures, one of the soul and one of the body. But distinguishing them and accepting their differentiation in thought only, we do not put their natures side by side nor, again, conceive of them as two separate natures ... Now when divine Scripture declared that “He suffered in the flesh” (1Pe 4:1) it is better that we speak thus and not of His “suffering in the nature of manhood”. ... we say that our Lord suffered in the flesh. So by the trick of the problem they say that He suffered in the nature of the humanity, distinguishing it from the Word and setting it on its own and apart , so that two are conceived of and not a single God the Word Who was incarnate and became Man.9
Saint Cyril was dealing with very tricky people and he had to be very cautious in the terminology he accepts. He always spoke of the "Flesh of Christ" rather than the "human nature of Christ", since the "flesh" does not imply duality while "human nature" implies separation of the natures after the union, which Nestorians advocated. Saint Cyril was not the first father who used the term One Nature for Christ:
For both natures are one by the combination, the Deity being made Man, and the Manhood deified.10
To maintain two natures in the one Christ, makes a Tetrad of the Trinity, ..... it is the true God, the unincarnate, that was manifested in the flesh, perfect with the true and divine perfection, not with two natures; nor do we speak of worshipping four (persons), viz., God, and the Son of God, and man, and the Holy Spirit.11
1. Saint Cyril of Alexandria: First Letter to Succensus, in John McGuckin: Saint Cyril of Alexandria and the Christological Controversy p 352.
2. Ibid p 353
3. Saint Cyril of Alexandria: The Letter to Acacius of Mitelene in Ebeid and Wickham: A collection of unpublished Syriac letters of Cyril of Alexandria, p 26
4. Ibid p 27
5. Saint Cyril of Alexandria: First Letter to Succensus, in John McGuckin: Saint Cyril of Alexandria and the Christological Controversy p 357.
6. Saint Cyril of Alexandria: Second Letter to Succensus, in Ebeid and Wickham: A collection of unpublished Syriac letters of Cyril of Alexandria, p 40
7. Ibid, p 42.
8. Ibid, p 41.
9. Ibid, p 43.
10. St. Gregory Nazianzen: Letter to Cledonius, Nicene and Post Nicene Fathers, Series II, Volume 5. read online
11. Gregory the Wonder worker: Sectional Confession of Faith, Ante Nicene Fathers, Volume VI. read online