“Since your Perfection enquires whether or not one ought to admit that there are two natures in Christ, I thought it necessary to address this point.” Thus does Saint Cyril define the reason of writing his First Letter to Succensus, bishop of Diocaesarea in Asia Minor (Turkey). Succensus was an orthodox and a good theologian, friendly to Saint Cyril, who alerted him to the views of the “Eastern bishops”, mostly sympathizers of Nestorius, and asked him to write refutations to what they propagated. Here is how this letter starts:

I read the memorandum sent by your Holiness and was most delighted that even though you are quite capable of bringing advantage both to us and to others from your own considerable learning, you saw fit to ask us to set down in writing what is in our mind, what we stand by. Well, we think the same things about the economy of our Saviour as the holy Fathers did before us. We regulate our own minds by reading their works so as to follow in their footsteps and introduce nothing that is new into the orthodox teachings.1

Here Saint Cyril assures his friend that what he will commit to writing is not his personal view, but rather what he had received from the fathers. Saint Cyril was very faithful to what he had received from Saint Athanasius, Saint Didymus and Saint Theophilus. He then proceeds to explain the Catholic faith:

We have learned from the divine scriptures and the holy Fathers to confess One Son, and Christ, and Lord. This is the Word of God the Father born from him in an ineffable and divine manner before the ages, and the same one is born from the holy virgin according to the flesh, for our sake, in the last times of this age. Since she gave birth to God made flesh and made man, for this reason we also call her the Mother of God. There is, therefore, One Son, One Lord Jesus Christ, both before the incarnation and after the incarnation. The Word of God the Father is not one distinct son, with the one born of the holy virgin being another and different son. No, it is our faith that the very one who was before the ages is the one who was born from a woman according to the flesh; not as if his Godhead took the beginnings of its existence or was called into being for the first time through the holy virgin, but rather, as I have said, that the eternal Word is said to have been born from her according to the flesh. For his flesh was his very own in just the same way as each one of us has his own body.2

And so, we unite the Word of God the Father to the holy flesh endowed with a rational soul, in an ineffable way that transcends understanding, without confusion, without change, and without alteration, and we thereby confess One Son, and Christ, and Lord; the same one God and man, not someone alongside someone different, but one and the same who is and is known to be both things. For this reason he sometimes speaks economically as man, in human fashion; and at other times, as God, he makes statements with divine authority. It is our contention that if we carefully examine the manner of the economy in the flesh and attentively investigate the mystery, we shall see that the Word of God the Father was made man and made flesh but did not fashion that sacred body from his own divine nature, but rather took it from the virgin. How else could he become man except by putting on the human body? As I have said, if we understand  the manner of the incarnation we shall see that two natures come together with one another, without confusion or change, in an indivisible union. The flesh is flesh and not Godhead, even though it became the flesh of God; and similarly the Word is God and not flesh even if he made the flesh his very own in the economy. Given that we understand this, we do no harm to that concurrence into union when we say that it took place out of two natures. After the union has occurred, however, we do not divide the natures from one another, nor do we sever the one and indivisible into two sons, but we say that there is One Son, and as the holy Fathers have stated: One Incarnate Nature of The Word. 
    As to the manner of the incarnation of the Only Begotten, then theoretically speaking (but only in so far as it appears to the eyes of the soul) we would admit that there are two united natures but only One Christ and Son and Lord, the Word of God made man and made flesh. If you like we can take as our example that very composition which makes us men. For we are composed of body and soul and we perceive two natures; there is one nature of the body, and a different nature of the soul, and yet one man from both of them in terms of the union. This composition from two natures does not turn the one man into two, but as I have said there is one man by the composition of body and soul. If we deny that there is one single Christ from two different natures, being indivisible after the union, then the enemies of orthodoxy will ask: “If the entirety amounts to one nature then how was he incarnated or what kind of flesh did he make his own?”3

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.

“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

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