The incarnation is a great mystery, transcending our capacities to comprehend. It entails the eternal Son taking into union a human nature, conceived by the Holy Spirit and born of the Virgin Mary. The biblical evidence for Jesus’s humanity is overwhelming, and there is no dispute about its reality. The text of John 1 and Luke 1 indicates that in becoming man, the Son or Logos remained who he eternally was and is. In taking human nature into permanent union, the Son grants eternal life to the human race.
For it was fitting for the creator of the universe, who by the economy of his incarnation became what by nature he was not, to preserve without change both what he himself was by nature and what he became in his incarnation… This is the great and hidden mystery, at once the blessed end for which all things are ordained. It is the divine purpose conceived before the beginning of created beings. In defining it we would say that this mystery is the preconceived goal for which everything exists, but which itself exists on account of nothing . . . a super-infinite plan infinitely preexisting the ages.
While the incarnation falls within the structures of our spatio-temporal humanity in this world, it also falls within the Life and Being of God—that is its astounding implication which needs to be thought out very carefully.
At the centre of the gospel is the incarnation. This dominating fact that “the Word became flesh and dwelt among us” (John 1:14) bestrides the pages of the New Testament. As Paul describes it, “in Christ God was reconciling the world to himself” (2 Cor. 5:19). Without the incarnation, we could not be saved. It was the incarnate Christ who died on the cross, rose from the dead, ascended into heaven, and is now at God’s right hand making intercession for us. In the words of Christopher Wordsworth,
You have raised our human nature, in the clouds to God’s right hand.
Aquinas wrote, “Nothing more marvellous can be thought of than this divine work: that the true God, the Son of God, should become true man.”
Underlying this mystery is the astonishing point that God alone could not save us. Putting it another way, apart from the Son’s coming among us as man, we could not be saved. This is not for any lack or deficiency on God’s part; he has the power to do as he pleases. But since he declares himself to be just, salvation by fiat would have been inconsistent with his character. The first question in our deliverance from sin and death was the satisfaction of God’s own justice. Being who he is, he could save us only in accordance with the dictates of his own character. His freedom is to act in harmony with who he is and not be constrained by anything external. Atonement for sin accords with divine justice. The sin of the first Adam could be atoned only by a second who did not bear the guilt and corruption inherited from the first. The Saviour needed to be man, a perfect and righteous man. This is expressed eloquently by the Heidelberg Catechism (1563):
Q. 15. What kind of mediator and deliverer should we look for then? A. He must be truly human and truly righteous, yet more powerful than all creatures, that is, he must also be true God.
Q. 16. Why must he be truly human and truly righteous?
A. God’s justice demands it: man has sinned, man must pay for his sin, but a sinner can not pay for others.
Q. 17. Why must he also be true God?
A. So that, by the power of his deity, he might bear the wrath of God in his humanity and earn for us and restore to us righteousness and life.
We must bear in mind the context of the incarnation. The human race was plunged into the mire of sin, guilty before God, and under his wrath. There was no other way for it to be delivered other than by the incarnation, life, atoning death, and ascension of the incarnate Son.
Maximus the Confessor summarizes the received teaching of the church:
The only-begotten Son, one of the holy and consubstantial Trinity, who is perfect God by nature, has become a perfect human being in accordance with his will, by truly assuming flesh that is consubstantial with us and endowed with a rational soul and mind from the holy Theotokos and ever-Virgin. He united it properly and inseparably to himself in accordance with the hypostasis,… Remaining God and consubstantial with the Father, when he became flesh, he became double, so that being double by nature, he had kinship by nature with both extremes, and preserved the natural difference of his own parts each from the other.
As Anselm argued against Roscelin, in Christ there is not One who is God and one who is a human being. Instead, the very same One who is the human being is God. The Son assumed into union a human nature, not a human person; the Son and the assumed human nature are the same person. While the Creator is infinite and the creature finite—the Creator-creature distinction remains inviolable—God has established a compatibility between himself and humanity, such that the humanity of Christ is the humanity of the eternal Son.
This is crucial. If the Creator-creature distinction applied absolutely, an infinite chasm would preclude that unbreakable union between God and humanity in Christ; we would end up with Nestorianism and could not be saved.
Since the Son found it possible to live as man, we must conclude that God made humanity in such a manner as to be capable of union with him in Christ. This is a vast mystery; human personality is analogous to the divine, neither identical or mingled nor alien. The creation of humanity, in which God breathes into Adam’s nostrils the breath of life (Gen. 2:7), and the final goal of participation in the divine nature (2 Pet. 1:4) point to this.
Following the Christological controversies, the church affirmed that the eternal Son had taken into union a human nature, body and soul, conceived by the Holy Spirit in the womb of the Virgin Mary. This is not a case of two separate natures coming together but rather the assumption into unbreakable union by the person of the Son of a human nature.
The most significant architect of this settlement was Cyril of Alexandria. He wrote of an ontological union that involved no change in the Son himself, for “the Word who is God came down out of heaven and entered our likeness . . . while ever remaining what he was.” The result is an “inseparable union,” “an indissoluble union.” Referring to Cyril’s writings before the Nestorian crisis ever arose, Weinandy puts it like this: for Cyril, “Jesus is one ontological entity, and the one ontological entity that Jesus is the one person of the Son of God existing as a complete and authentic man.” So “the incarnational act does not bring about a union of natures, but rather it is an act by which the humanity is united substantially to the person of the Word.” Jesus is the same Son who existed eternally with the Father and who came to exist as man. Hence, in terms of his personal identity—who he is—he is the eternal Son, one of the Trinity according to the flesh, and his humanity—both body and soul—is that of the Son.
Content taken from Systematic Theology by Robert Letham, ©2019. Used by permission of Crossway, a publishing ministry of Good News Publishers, Wheaton, Il 60187, www.crossway.org. You can buy it online here.
 Maximus the Confessor, Ad Thalassium 60, in On the Cosmic Mystery of Jesus Christ: Selected Writings from St Maximus the Confessor, trans. Paul M. Blowers and Robert Louis Wilken (Crestwood, NY: St Vladimir’s Seminary Press, 2003), 124–25.
 Thomas F. Torrance, The Christian Doctrine of God: One Being, Three Persons, 2nd ed. (Edinburgh: T&T Clark, 2016), 95.
 From the hymn “See the Conqueror Mounts in Triumph” (1862).
 Aquinas, SCG 4.27.1.
 Maximus,Opusculum7(PG,91:73B-C),citedinChristopherA.Beeley,TheUnityofChrist: Continuity and Conflict in Patristic Tradition (New Haven, CT: Yale University Press, 2012), 297.
 Anselm, De fide 11 (PL, 158:279)
 Sergius Bulgakov, The Wisdom of God: A Brief Summary of Sophiology (London: Williams and Norgate, 1937), 126–31. Bulgakov was highly speculative and controversial, but in this he has some important things to say, among others.
 Cyril of Alexandria, On the Unity of Christ, trans. John Anthony McGuckin (Crestwood, NY: St Vladimir’s Seminary Press, 1995), 61 (also PG, 75:1269); Thomas G. Weinandy, “Cyril and the Mystery of the Incarnation,” in The Theology of St Cyril of Alexandria: A Critical Appreciation, ed. Thomas G. Weinandy (London: T&T Clark, 2003), 27–28.
 Cyril, The Unity of Christ, 77 (PG, 75:1289).
 Weinandy, “Cyril,” 30.
 Weinandy, “Cyril,” 41.
 Weinandy, “Cyril,” 43; See Cyril, The Unity of Christ, 107–16.
Robert (Bob) Letham is Professor of Systematic and Historical Theology at Union School of Theology. He is the author of numerous theological books, including The Work of Christ in the Contours of Christian Theology series.