3.2 summary from a secondary source

I excerpted this from Knowing the Love of Christ: An Introduction to the Theology of St. Thomas Aquinas, by Michael Dauphinais & Matthew Levering. This section in the Summa is a brain-bender, at least for me. Hopefully the following overview will help.

Just before Jesus' death and resurrection, Philip asked Jesus, "Lord, show us the Father, and we shall be satisfied" (Jn 14:8). Jesus answered him, "Have I been with you so long, and yet you do not know me, Philip? He who has seen me has seen the Father." It is crucial to recognize, before we proceed, that the speculative analysis involved in understanding the Incarnation - that is, the "hypostatic union" of the divine and human natures in the Person of the Son - is simply a way of articulating what is meant by Jesus' declaration that whoever has seen him has seen the Father.

... Now we turn to the question of who Christ has to be in order to accomplish [our salvation.] ... We must guard against a distaste for the intricacies of Christological doctrines. In the centuries after Christ's death and resurrection, the Church never shied away from facing such doctrinal difficulties because she knew that if we misunderstand who Jesus is, then the reality of his salvation will be closed to us.

Following the creeds of the Church as formulated by early councils, St. Thomas holds that Jesus Christ possesses two natures, one human and one divine. Jesus Christ, however, only is one Person, the divine Person of the Son of God. The distinction between nature and person is recognized in everyday speech. If a child sees someone walking down the street and asks, "What is that?" the parent says, "That's a man." If the child asks, "Who is that?" the parent says, "That's Uncle Charlie." The concept "nature" answers the question "What is it?" and the concept "person" answers the question "Who is it?" When the disciples saw Jesus and asked, "What is Jesus?" both answers would be correct, "That's a man" and "That's God." When the disciples asked, "Who is Jesus?" the answer would be "That's the Son of God, the Second Person of the Trinity." [cf Mt 16:15-16] ...

It remains a mystery - something above, not contrary to, human reason - how the Second Person of the Trinity was born of a virgin, lived, suffered, died, and rose again. This incomprehension, however, does not require that we turn off our intellects. The reverse is the case. Because the divine nature is beyond our comprehension, it can take to itself a human nature without ceasing to be divine.

Consider it this way. Try to imagine a dog-man who was fully canine and fully human. You cannot. The reason why the two natures are incompatible with each other is that they are both created natures. As created natures they occupy the same field of action. In this way, they are in a competitive relationship - either a dog or a man, but not both. Now consider the union of two natures, one divine and one human. Here there is a noncompetitive relationship because the divine nature is the source of the human nature, just as it is the source of every other created nature. Our inability to comprehend the divine nature stems from its distinction from creation.

Once this distinction between the Creator and his creation is recognized, then the Incarnation ceases to be a contradiction. It is a miracle but not a logical impossibility. One being can be both God and man because the two natures do not exist side by side. Many of the early hersies about Christ viewed the divine and the human natures waging a turf-battle in the person of Jesus Christ. Many modern objectors to the Incarnation likewise assert that it is simply impossible for one thing to be both God and man. These objections err by treating the divine nature as simply another nature within the universe. Once God is confessed as the Creator then the Incarnation can be confessed in a way that avoids contradiction.

... This understanding of Jesus Christ allows us to speak correctly about him and to interpret correctly his deeds and words in the Gospels. It is true to say, as one of the early councils did, that "one of the Trinity suffered and died on the Cross." The Person of the Word is the only Person in Jesus Christ. The Person of the Word, therefore, is the only subject to whom we can attribute Christ's action. If someone asks how can God suffer since God is all-powerful and unchanging, then we can answer that the Word suffered not in his divine nature, but in his human nature. We also see in the Gospels Jesus performing great miracles over the physical forces of nature - the calming of the stormy sea and the raising of Lazarus. His human nature of itself had no power to perform these miracles. Yet his divine nature surely could do so. But the two natures cannot be separated since they are united in one person. In this way, whatever is done by the divine nature can be said of the human nature and vice versa.

... Christ's divinized human nature thus stands as the pattern after which we will also be divinized. Our divinization, however, is of a lesser order than Christ's since we become children of God not by nature, but by the adoption of grace. Nonetheless, the Incarnation of the Word makes possible the supernatural goal of human life, namely, sharing in God's own life.