The crucial point, however, is that in the Nestorian way of thinking, Jesus is the human nature in Christ and is therefore not himself identical to the Divine Logos. This latter point is what Justinian has in mind when he makes his charge, for with St. Cyril he wishes to emphasize that Jesus is not someone else than the Divine Logos; “Christ” in other words, is the Divine Logos only who as the incarnate Divine Logos is both human and divine in nature, but divine only in identity or person.
It is important to note how this view of Christ’s particularity distinguishes Justinian’s “Cyrillian” Chalcedonianism from Nestorianism and from many Christologies one encounters in Western Christian thought. At issue is “who” lies inside the particular prosopon of Christ, and what is the starting point for determining that. Both Nestorianism and Cyrillian Chalcedonianism acknowledge that there is one Christ who is one particular or hypostasis or prosopon, and that furtermore this one Christ is divine and human in his natures. Many contemporary theologians who have sought to vindicate Nestorius from his condemnation at the Council of Ephesus in 431 base their defense of Nestorius precisely on this point: Nestorius, as also the Council of Chalcedon in 451, taught that Christ is one particular who is both God and man. But many of these scholars fail to grasp the significance of the fundamentally different starting points characterizing these two Christologies which lead to radically different notions of hypostasis and the content and identity of Christ. F. Loofs, perhaps, remains one of the most perceptive students of Nestorius, for he recognizes that if one is to uphold Nestorius’ Christological understanding, one must reject Cyril’s. And it is noteworthy that those who seek to reconcile Nestorius and Cyril must do so apart from Cyril’s 12 Anathemas against Nestorius, which capsulize the core of Cyril’s thought and were the source of conflict with the defenders of Nestorius even in the 5th century. Loofs placed his finger on the heart of the matter when he wrote:
“What does Nestorius mean when he talks about the one prosopon of Christ? The undivided appearance [Wesche’s italics] of the historic Jesus Christ. For he says, very often, that Christ is the one prosopon of the union. And he argued with Cyril: ‘You start in your account with the Creator of the natures and not with the prosopon of the union. It is not the Logos who has become twofold; it is the one Lord Jesus Christ who is twofold in his natures.’”
In other words, as Nestorius himself observed, Cyril—and Justinian—starts from “inside” the prosopon of Christ, from the Divine Logos. Nestorius, on the other hand, and the theologians who share his Christological perspective, start from “outside” of Christ, i.e. from that which can be visibly seen, the “undivided appearance” or prosopon. These different starting points yield radically different confessions concerning the philosophical content of the particular or hypostasis of Christ: the former understands hypostasis in terms of identity, i.e. the subjective one, the “self” (autos in Greek) or “who” of Christ, which is one, and is seen to be the Divine Logos himself so that the terms “Jesus,” “Christ,” and “Divine Logos” are identical, referring to one and the same subject. The hypostasis,then, is the foundation, not the product, of the union, for it is the eternally existing Divine Logos, the one through whom all things came into being in the first place. The latter, on the other hand, starting from the “undivided appearance” of the historical Jesus, understands hypostasis as the product rather than the foundation of the coming together of the two natures. These two natures, moreover, are each seen as two fully intact subjects: Jesus is the human nature and so is a “someone other” than the Divine Logos, for the Divine Logos is the divine nature.
On the basis of this Cyrillian Christology Justinian published the condemnation of the Three Chapters in 543, which was confirmed by the Fifth Ecumenical Council in Constantinople in 553. The Three Chapters were “Nestorian” documents from the late fourth and fifth centuries.
Rev. Kenneth P. Wesche, On The Person Of Christ: The Christology of Emperor Justinian (SVS Press), pp. 17-19
Bold italics are my own emphasis.
 See, for example, J. F. Bethune-Baker, Nestorius and his Teaching (Cambridge, 1908); Friedrich Loofs, Nestorius and his Place in the History of Christian Doctrine (Cambridge, 1914); and Milton V. Anastos, “Nestorius was Orthodox,” in Dumbarton Oaks Papers 16 (1962) 118-140. For further bibliographical information see J. Quasten’s Patrology, vol. 3, The Golden Age of Greek Patristic Literature (Westminster, Maryland: Christian Classics, Inc. 1983), and note 1 of Anastos’ study.
 The Anathemas are contained in Cyril’s third letter to Nestorius, the text which is translated into English in E. R. Hardy, Christology of the Later Fathers, pp. 353f; and also in the NPNF 2nd series, vol. 14, pp. 206-218 (with notes).
 Loofs, op. cit., p. 79. Loofs repeats the same point elsewhere when he defends Nestorius in this way: “Still more intelligible does the Christology of Nestorius become to us if, following his advice, we start from the one prosopon of the union, i.e. from the one Jesus Christ of history” (p. 93).