In the third part of the Summa Theologiae, commonly referred to in Latin as the tertia pars, Thomas Aquinas turns his attention to the topic of Jesus Christ. After discussing the fittingness of the Incarnation (q. 1), Aquinas then turns to considering the mode of the union of the Word. This, in other words, examines the way in which the Son was united to a human nature and, as such, became incarnate. One easy-sounding question in this topic is whether Christ is a Divine Person with a human nature, or a human person with a Divine Nature.
To be clear, both choices accept that Jesus is both human and Divine, so it seems that both options are equally valid. This, however, is not the case, but in order to understand why this is not the case one must understand some finer points regarding the metaphysics of the Incarnation.
‘Nature’ describes what a thing is. A human has human nature or “humanness” and a cat has cat nature or “catiness.” Nature is the answer to the question of what something is, fundamentally. All beings that exist subsist under one nature. The Father subsists under the Divine Nature. There is only one exception to this rule, the Son. Unlike any other being, the Son has subsisted both under the Divine Nature and human nature since the moment of the Incarnation.
This brings about a unique issue. For all other beings, to say that x is a particular instance of a certain nature is to say that that being has its own act of existence. A particular instance of the human nature, for example, is a human being. If this holds true for Christ and He has two acts of existence, this entails that Christ is actually two persons, a Divine Person and a human person. A ‘person’ is a subset of ‘being,’ which denotes a being endowed with rationality. As far as we know, humans, angels, and God are persons, whereas animals and plants are not. A person, therefore, is a rational nature actualized by a particular act of existence.
This position—that Christ is made up of two persons—is known as Nestorianism and was condemned at the Council of Ephesus. The Council held that if God the Son was not truly united to Jesus Christ, then His death and Resurrection would not ensure salvation, by the famous principle of St. Gregory of Nazianzus, “What [the Word] did not assume, He did not redeem.” Thus, if the Word did not fully unite Himself with what is properly human in each of us, He did not save us.
Finally, God is eternal. That means that the Son existed in eternity past, long before He was Incarnated by the Holy Spirit and the Virgin Mary. That means that He already possessed His act of existence. In fact, it is a bit more complicated than that. All things in existence have their particular nature—more commonly referred to in this case as essence—and the act of existence. God, however, does not have an essence, properly speaking. Aquinas points out that God’s essence is His existence. In all other beings, the act of being is limited by essence, whereas God is unlimited Being. Thus, God is perfectly simple, He is not made of parts whatsoever.
Now, let us combine all the principles. The Son existed in eternity past and thus has—indeed He is—His own act of existence. A human person is essence actualized by existence. If Christ, then, were a human person with a Divine Nature, He would necessarily have to be two persons, one human and one Divine. This position, however, is tantamount to Nestorianism, which is heretical. It must be, therefore, that Christ is a Divine Person with a human nature.
One might ask, however, why this option works and why it is that Christ is the only being in existence to subsist under two natures. The reason is precisely because the Son is—rather than has—His own act of existence. Since God the Son is pure existence, He can actualize the human nature united to the Son. Christ’s union, therefore, is something that only God can do.
Of course, this may seem rather complicated and philosophical-sounding. Can we even suppose that anyone in the time of Christ understood the Incarnation in this way? I submit that St. Paul is saying precisely this when he says, “Have this attitude in yourselves which was also in Christ Jesus, who, although He existed in the form of God, did not regard equality with God a thing to be grasped, but emptied Himself, taking the form of a bond-servant, and being made in the likeness of men” (Phil. 2:5-7).
A close reading of that passage reveals that Paul: Asserted that the Son existed in the form (one could here substitute the term ‘nature’) of God (v. 6) and that He emptied Himself—the doctrine known as the kenosis—by taking on the form (i.e. nature) of a bond-servant (i.e. human) (v. 7), which was the conclusion above. Thus, we can see that this understanding of the Incarnation was present in the Church from the beginning, though it must be said that Thomas’ exposition—although by no means easy—provides an easy framework in which to understand it.