“We say that there is one Son, and that he has one nature even when he is considered as having assumed flesh endowed with a rational soul” (On the Unity of Christ, p. 77; my emphasis). We first note the assertion of the one Son: Jesus Christ is identical to the eternal Son of God the Father. St Cyril thus reaffirms the unitive thrust of his christology and rejects any suggestion of Jesus being a creaturely son alongside the divine Son. Jesus simply is the Son. “You must not divide him who is of David’s line by saying that he is someone different to the one Christ and Son and Lord,” writes Cyril, “for the correct position is that the Only Begotten Son who is born from God the Father is himself, and no other, the Son of David according to the flesh” (p. 83). Or as Christopher Beeley puts it: “There is one Son in the incarnation just as there is one Son apart from it” (The Unity of Christ, p. 260). But then Cyril goes on to speak of the “one nature” of Christ after the Incarnation. The heirs of Chalcedon do not speak this way. We speak of the one hypostasis of the incarnate Son in two natures. Cyril himself frequently spoke of the Incarnation as a “union in hypostasis,” and he insisted upon the theandric union as the consolidation of divinity and humanity, without confusion or mixture. So why speak of “one incarnate nature”?
To clarify his meaning Cyril offers a creaturely analogy. “Do we not say,” he asks, “that a human being like ourselves is one, and has a single nature, even though he is not homogeneous but really composed of two things, I mean body and soul?” (p. 78; my emphasis). When body and soul are joined together to make a human being, they do not lose their distinctiveness nor is their integrity compromised. At first glance the analogy seems weak. The union of soul and body brings about a composite being greater than its parts; but in the hypostatic union the divine Son remains the divine Son, only now enfleshed.
Let’s take a look at Cyril’s two letters to Succensus, written sometime between 434-438, to see if we can gain further clarity.
In his first letter Cyril reiterates his contention that in the Incarnation “two natures come together with one another, without confusion or change, in an indivisible union” (Ep. 45.6). He then makes the following important statement:
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. (Ep.45.6-7)
The Incarnation occurs out of or from two natures, resulting in the one incarnate nature of Christ. At an intellectual or notional level, we may of course distinguish the divine and human natures in Christ—neither are obliterated when hypostatically united—but in reality the two natures have so interpenetrated the other that there is now only the single nature of the God-Man. Cyril restates this argument in his second letter, in response to a criticism of his one-nature formulation:
This objection is yet another attack on those who say that there is one incarnate nature of the Son. They want to show that the idea is foolish and so they keep on arguing at every turn that two natures endured. They have forgotten, however, that it is only those things that are usually distinguished at more than a merely theoretical level which split apart from one another in differentiated separateness and radical distinction. Let us once more take the example of an ordinary man. We recognise two natures in him; for there is one nature of the soul and another of the body, but we divide them only at a theoretical level, and by subtle speculation, or rather we accept the distinction only in our mental intuitions, and we do not set the natures apart nor do we grant that they have a radical separateness, but we understand them to belong to one man. This is why the two are no longer two, but through both of them the one living creature is rendered complete.
And so, even if one attributes the nature of manhood and Godhead to the Emmanuel, still the manhood has become the personal property of the Word and we understand there is One Son together with it. The God-inspired scripture tells us that he suffered in the flesh (1 Pet. 4.1) and it would be better for us to speak this way rather than [say he suffered] in the nature of the manhood, even though such a statement (unless it is said uncompromisingly by certain people) does not damage the sense of the mystery. For what else is the nature of manhood except the flesh with a rational soul? (Ep. 46.5)
Again we see Cyril allowing a distinction between the two natures at a theoretical level, but he has little interest in dualistic theories about Christ. What is important is the one divine Son who has clothed himself in our humanity. He especially fears that the Nestorian rhetoric of an abiding association of two natures must ultimately dissolve the Incarnation. “Cyril is happy,” John McGuckin explains, “to accept the notion of ‘two natures’ but feels that this needs qualification if it is to avoid a tendency towards the kind of separatism that has been advocated by Nestorius. He wishes to speak of a concurrence to unity ‘from two natures’ but does not posit a union that abides ‘in two natures’. For Cyril, to abide in two natures means to abide in an ‘un-united’ condition that can only be theoretically applied before the incarnation takes place; the incarnation itself is the resolution to union of the two natures” (Saint Cyril of Alexandria and the Christological Controversy, p. 355, n. 6).
At stake is the single-subject hermeneutic discussed in “Holy Scripture and the Grammar of the Son.” But ultimately what is at stake is nothing less than the salvation of the world, for if Jesus is not the Word himself, all is undone. McGuckin elaborates upon the importance of the mia physis for Cyril:
For Cyril, if the christological union means anything it means that there is only one reality to be affirmed henceforth. This concrete reality (physis) is what stands before the christian observer; it is a single concrete reality enfleshed before us: Mia Physis Sesarkomene. What is more, that concrete, fleshed-out reality, is that of the Word of God, none other. In short, by using the phrase Cyril is attributing the person of the Word as the single subject of the incarnation event. He does so in a phrase which is highly succinct (a good rallying phrase for his party), provocatively robust (using concrete physis terms as opposed to the semantic word-plays of Nestorius), and radically insistent on the single subjectivity of the divine Word (the direct personal subject of the incarnate acts). (p. 209)
For Cyril physis signifies a “concrete personal individuate”—thus equivalent to hypostasis. He employs both terms to refer “to individual and real personal subjectivity, and in the way he uses them they are synonyms of ‘the Logos as subject'” (p. 209). As Cyril writes: “Thus, there is only one nature (physis) of the Word, or hypostasis if you like, and that is the Word himself” (quoted by McGuckin, p. 209). Yet given that physis can also signify an entity’s defining properties (see Hans van Loon, The Dyophysite Christology of Cyril of Alexandria [pp. 512-517]), one can understand why Antiochenes and Latins found the “one incarnate nature” objectionable and why the phrase did not survive in the post-Chalcedonian Church.