I recently finished reading On the Unity of Christ by St Cyril of Alexandria. Somewhat surprisingly, I found it slow-going—not because of its philosophical difficulty but rather its exegetical density. Cyril is a biblical theologian, and he would counter the heresy of Nestorius with a comprehensive reading of the Scriptures. He believes that the Nestorians have interpreted Scripture through a dualistic hermeneutic and for this reason have grievously distorted the catholic understanding of the Incarnation and rendered the evangelical narrative incoherent. How is the eternal Word united to the man Jesus? Does the Word assume a human being in the womb of Mary, thus necessitating a conjunction of Word and Jesus (resulting in two agents), or does he actually become a human being, with body and soul (resulting in one incarnate agent)? If the former, then Jesus is but a prophet—perhaps a prophet perfectly indwelt by the Word and thus intimately associated with the divine Sonship but ultimately not any different than believers who have been adopted as sons through baptism. Cyril puts it like this, in the dialogical form that characterizes the tract:
A. Doubtless we will find their way of thinking to be offering us two sons and two Christs?
B. No, not two. They say that the Word of God the Father, who is Son by nature, is one; but that the man who is assumed is by nature the Son of David, and is the Son of God insofar as he is assumed by God the Word. They say that he has come to such dignity and has the sonship by grace, because God the Word dwells within him.
A. What happened to their brains and their intelligence—people who hold such opinions? How can they possibly say there is not a duality of sons when they split off man and God from each other? If, as they say, one is truly the Son by nature, but the other has the sonship by grace and came to such dignity because of the Word dwelling with him, then what more does he have than us? For the Word also dwells in us. For the most holy Paul confirms this point for us when he says: “For this reason I bend my knees before the Father, from whom all fatherhood is named in heaven and on earth. May he grant that you are strengthened in power through his Spirit, according to the riches of his glory, that Christ may dwell in your hearts” (Eph 3:14-17); and he is within us by the Spirit, “in whom we cry out Abba, Father” (Rom 8:15). And so, if we have been granted the same dignity by God the Father, our position is in no way inferior to his. For we too are sons and gods by grace, and we have surely been brought to this wonderful and supernatural dignity since we have the Only Begotten Word of God dwelling within us. It is completely wicked and foolish for them them to say that Jesus has been granted the dignity of the Sonship and has won this glory as a matter of grace. (pp. 79-80)
On the Unity of Christ demonstrates, in the words of Christopher A. Beeley, Cyril’s “clear commitment to the practice of single-subject biblical interpretation” (The Unity of Christ, p. 264). When interpreting the Scriptures, we must eschew any separation between the divine Son and the son of Mary: they are one indivisible subject and agent. There is only one incarnate Word, and of him we may simultaneously predicate divine and human properties and actions:
B. Then must we confess that it [the flesh of Jesus] has become entirely the personal body of the Word of the Father with no one else intervening, even though it is understood as animated with a rational soul?
A. Exactly so, if we are to define the doctrine of faith correctly and without error, and are lovers of the doctrines of the truth, who follow in the track of the faith of our holy fathers. … This is why we believe that there is only one Son of God the Father. This is why we must understand Our Lord Jesus Christ in one person [prosopon]. As the Word he is born divinely before all ages and times, but in these last times of this age the same one was born of a woman according to the flesh. To the same one we attribute both the divine and human characteristics, and we also say that to the same one belongs the birth and the suffering on the cross since he appropriated everything that belonged to his own flesh, while ever remaining impassible in the nature of the Godhead. This is why “every knee shall bend before him, and every tongue shall confess that Jesus Christ is Lord, to the glory of God the Father” (Phil 2:10-22). (p.133)
Seven years earlier, Cyril had personally instructed Nestorius on this fundamental grammatical principle:
We do not divide out the sayings of our Saviour in the Gospels as if to two hypostases or prosopa. The one and only Christ is not twofold even though he is understood as compounded out of two different elements in an indivisible unity, just as a man is understood as consisting of soul and body and yet is not twofold but rather is one from out of both. No, we think correctly and so we must maintain that both the manly as well as the godly sayhings were uttered by one subject. When he speaks of himself in a God-befitting way: ‘Whoever has seen me has seen the Father’ (Jn. 14.90), and ‘I and the Father are one’ (Jn. 10.30), we are given to understand his divine and ineffable nature in which he is one with his own Father by identity of essence, the ‘image and impress and effulgence of his glory’ (cf. Heb. 1.3). On the other hand when, not despising the limitations of the manhood, he speaks to the Jews: ‘Now you seek to destroy me; a man who has told you the truth’ (Jn. 8.40), we nonetheless recognize him as God the Word in the equality and limitations of his manhood. … This is why all the sayings in the Gospels are to be attributed to one prosopon, and to the one enfleshed hypostasis of the Word, just as according to the scriptures there is One Lord Jesus Christ (1 Cor. 8.6). (Third Letter of Cyril to Nestorius 8)
The grammatical rule can also be formulated in negative form:
If anyone interprets the sayings in the Gospels and apostolic writings, or the things said about Christ by the saints, or the things he says about himself, as referring to two prosopa or hypostases, attributing some of them to a man conceived of as separate from the Word of God, and attributing others (as divine) exclusively to the Word of God the Father, let him be anathema. (3Ep.Nest. 12 anathema 4)
Cyril thus promulgates the kind of “partitive exegesis” exemplified by St Gregory of Nazianzus and other Nicene theologians in the fourth century:
For the Nicenes, on the other hand, Scripture speaks throughout of Christ, but the Christ of the kerygma, the crucified and exalted Lord, and speaks of him in a twofold fashion, demanding in turn a “partitive” exegesis: some things are said of him as divine and other things are said of him as human—yet referring to the same Christ throughout. (John Behr, The Nicene Faith, p. 14)
Latin theologians would later term this exegetical practice as communicatio idiomatum.