An excellent question that we get here at Catholic Answers maybe five or six times a year is this:
If at the Incarnation “the Word was made flesh,” doesn’t that mean that God changed? And if he changed, how can he be God?”
Because this question is connected to Mary’s title Theotokos (“God-bearer), I address it at length in my book, Behold Your Mother. (Check it out!) But this is such a good and important question that it bears treating over and over again.
The short answer is no, God cannot change (see Mal. 3:6), so he did not change in the Incarnation. In order to understand how this is so, we have to define a very important term: the hypostatic union. At the Incarnation 2,000 years ago, the second person of the Blessed Trinity acquired a human nature; ever since then, the second person of the Blessed Trinity possesses two natures, one divine, and one human, subsisting within the one divine person. (End of definition.)
Please take note that there is no “mixing” of natures here. The divine and human natures of Christ are absolutely distinct. Yet they are not divided or separated, either: they are “joined” in the hypostatic union—a created union “within” the one divine hypostasis (person) of Christ.
What do we mean by this union existing “within” the one divine person? That seems to imply a change in God, right?
St. Thomas Aquinas explains:
Since the divine Person is infinite, no addition can be made to it: Hence Cyril says [Council of Ephesus, Part I, ch. 26]: “We do not conceive the mode of conjunction to be according to addition”; just as in the union of man with God, nothing is added to God by the grace of adoption, but what is divine is united to man; hence, not God but man is perfected (Summa Theologiae, Pt. III, Q. 3, Art. 1, Reply Obj. 1).
When we speak of the hypostatic union of the divine and human natures of Christ in the one divine person, we have to qualify what we mean by the union being “in the person” of Christ. St. Thomas explains that since the hypostatic union is a “created union,” it cannot be “in God”:
[E]very relation which we consider between God and the creature is really in the creature, by whose change the relation is brought into being; whereas it is not really in God, but only in our way of thinking, since it does not arise from any change in God. And hence we must say that the union of which we are speaking is not really in God, except only in our way of thinking; but in the human nature, which is a creature… Therefore we must say it [the hypostatic union] is something created (Ibid., Q. 2, Art. 7).
And so when the councils of Ephesus and Chalcedon speak of the hypostatic union being “in the person of Christ,” it is so inasmuch as the human nature assumed by Christ now has as its subject the divine person of Christ. As Aquinas says, it is “not really in God, except in our way of thinking”—that is, because the human nature has as its subject a divine person.
“But wait a minute,” someone might say. “The second person of the Blessed Trinity used to be just God, but at the Incarnation he became a God/man mixture. That’s a change!”
It’s true that, at the Incarnation, the second person of the Blessed Trinity added a human nature that he did not have before, but he (the divine person) did not change in his divine essence. The only real change took place in his human nature, which received infinite dignity in and through the hypostatic union but God did not change in the process. But because of the hypostatic union, when one refers to the human nature of Christ, the subject is the divine person. This is why we can worship the man, Jesus Christ. This is why we can affirm that God, the second person of the Blessed Trinity, was born, suffered, and died. The divine nature cannot die, but a divine person did, because of the hypostatic union.
Likewise, we also worship the whole Christ, not part of him. Mary gave birth to the whole Christ, not to one of his natures. When we speak of Jesus we speak of his divine person, to which everything fully human and divine is attributed.
This is a great mystery, and we should not shy away from admitting it. 1 Timothy 3:16 says, “Great is the mystery of godliness. He [God] was manifest in the flesh, seen of angels preached on unto the gentiles and received up into glory.” The truth of the hypostatic union is beyond our ability to comprehend fully, but there is nothing about it that is contrary to reason.
But to posit a change in one of the three divine persons of the Blessed Trinity is clearly contrary to reason. And that is no mystery at all. The Council of Ephesus (431) affirmed by the later Council of Chalcedon (451), says quite succinctly and accurately:
He did not cast aside what he was, but although he assumed flesh and blood, he remained what he was, God in nature and truth. We do not say that his flesh was turned into the nature of the godhead or that the unspeakable Word of God was changed into the nature of the flesh. For he (the Word) is unalterable and absolutely unchangeable and remains always the same as the scriptures say (Mal. 3:6).
Even the heretical Nestorians, who effectively saw two persons in Christ, did not make so obvious a mistake as to claim there was “change” in God.
Ultimately, the error in those who think that God changed in the Incarnation is rooted in a lack of understanding of the difference between person (who someone is) and nature (what someone or something is). In Jesus’ case, the “who” does not necessarily change by adding another “what.” And neither is there a change to the other “what”—the divine nature. The only change we can speak about is the radical change to human nature that occurs by the grace of the hypostatic union wherein the human nature of Christ is lifted up, as it were, into the divine person, receiving infinite dignity as a result. Thus, “the man, Jesus Christ” (1 Tim. 2:5) has not merely become a “[partaker] of the divine nature” (2 Peter 1:4) as we Christians are. The “man” Jesus Christ is God because of the hypostatic union.
And isn’t this exactly what we see in Sacred Scripture? Examine Colossians 1:15-22 and you will see the “he,” or the one person of Christ, being referred to as both “the creator” of all things (as God) and as having suffered and died on the cross for our salvation (as man):
He is the image of the invisible God, the first-born of all creation; for in him all things were created, in heaven and on earth, visible and invisible, whether thrones or dominions or principalities or authorities — all things were created through him and for him. He is before all things, and in him all things hold together. He is the head of the body, the church; he is the beginning, the first-born from the dead, that in everything he might be pre-eminent. For in him all the fullness of God was pleased to dwell, and through him to reconcile to himself all things, whether on earth or in heaven, making peace by the blood of his cross. And you, who once were estranged and hostile in mind, doing evil deeds, he has now reconciled in his body of flesh by his death, in order to present you holy and blameless and irreproachable before him (emphases added).
He is both God the creator and the man who died on the cross. How? Because any phenomenon of which we speak, finding its source, location, or both in Christ, must ultimately be attributed to the one divine person. This is why we can say with confidence that God became man yet God does not change.