Aquinas on Incarnation and Christmas

The ultimate mystery and meaning of Christmas, and really the entire Christian faith, is the Incarnation. It is the very heart of all Christian creeds. But it is, in its beautiful, wondrous depth, unfathomable to finite minds. What impenetrable profundity, to say that God became man, that the Divine Reality took on flesh!

Such a feat would not be really so problematic in many mythologies/theologies. Just read through the literature of many ancient myths, and you’ll see that the gods are understood really as just “glorified” (in a very ironic sense) humans–immortal, much more powerful, but in essence still all too human. These gods, in effect, differ from mankind merely by degree. They are “higher on the scale” in terms of qualities.

But nothing could be more different from the belief of Classical Theism, which holds God to be not just another being amongst all beings, with a little more power, or even a “higher” being; but rather Being Itself, Existence Itself, Pure Actuality, Goodness Itself, etc. That in whom Essence and Existence are identical, totally simple (no metaphysical composition), completely and totally unchanging and unchangeable, absolutely no passive potencies, qualitatively infinite in power and goodness and knowledge, entirely immaterial, incorporeal, impassible, not able to suffer or be affected externally. This is the God, acknowledged in the Jewish and Muslim faiths as well, but who the Christian faith alone of all traditions has declared became a human being, like us, to walk among us, live beside us, and die for us.

But how, in a metaphysical sense, is this even possible? How can something that by its very nature is Pure Act, Being Itself, immutable, impassible, immaterial, and infinite, become something finite, material and physical, changeable, a being among beings, composite of act and potency? In short, how is the Incarnation not a total contradiction in terms?

The orthodox Christian doctrine, established in the Council of Chalcedon, is that of Hypostatic Union: “in Christ one person subsists in two natures, the Divine and the human” [1].

The metaphysics of this Hypostatic Union is a topic St. Thomas addresses extensively in the Pars Tertia of his Summa Theologiae. In the First Article of the Second Question of this Part, he asks “Whether the Union of the Incarnate Word Took Place in the Nature?” [2], meaning: did the union of Christ’s two natures take place in one of the natures itself? To which his answer is negative. This will be a summary of his arguments.

First he defines what a “nature” is: “We are now speaking of nature as it signifies the essence, or the ‘what-it-is,’ or the quiddity of the species” [3]. In short, the nature refers to the essence of the being. Next he considers in what ways one thing can be made up of different things. There are three ways that this is so, he says. The first is “from two complete things which remain in their perfection. This can only happen to those whose form is composition, order, or figure” [4]. In other words, objects such as houses or piles of stone, in which each individual part remains essentially and wholly itself, but is just configured or placed together into the figure of another object. But he rejects this as the manner of the union of Christ’s two natures. For in such a composition, the “form” comes only accidentally. A pile of stones is not actually a being in its own right, it just consists of several distinct beings who happen to have been placed together. The same is true of a house, which is just “made of stones and beams arranged in order” [5]. And if this were so, “it would follow that the union of the Incarnation was not essential, but accidental” [6], and this is impossible (he proves this later on). Furthermore, in such a composition, “we should not have an absolute unity, but relative only, for there remain several things actually” [7]. But the Hypostatic Union is an absolute unity.

Another way one thing can be made up of several is when the several different things are “perfect but changed” [8]. The example he gives here is “as a mixture is made up of its elements” [9]. Think of a chemical molecule whose bonded atoms share electron pairs, thus undergoing “change” within themselves. But this cannot be the manner of the Union, “because the Divine Nature is altogether immutable . . . hence neither can it be changed into something else, since it is incorruptible; nor can anything else be changed into it, for it cannot be generated” [10]. Since the union involves the Divine Nature, and the Divine Nature can admit to absolutely no change or alteration, the Union cannot be such that it requires any change within its natures. In addition, he gives another reason:

“There can be no mingling of things widely apart; for the species of one of them is absorbed, e.g. if we were to put a drop of water in a flagon of wine. And hence, since the Divine Nature infinitely exceeds the human nature, there could be no mixture, but the Divine Nature alone would remain” [11].

Things so vastly different and opposed in essence cannot be mixed, and since the Divine Nature so supersedes the human in terms of infinite power, goodness, existence, etc., the human nature would be utterly dissolved, leaving only the Divine Nature.

The last way a thing can be made up of different things is when it “is made up of things not mixed nor changed, but imperfect; as man is made up of soul and body, and likewise of diverse members” [12]. On Aristotelian hylomorphism, everything consists of both “form” and “matter” together. Anything material cannot exist without both, and thus each alone is “imperfect”, not sufficient for actual existence. It is only together that they make up a full, complete being. But this again cannot be the manner of the Union. For in the Union, each individual Nature is already complete in its own right. A human essence can exist on its own, as a human being; the Divine Nature can exist on its own, as God Himself.

So the Union of Christ’s two natures cannot happen in one of the natures. St. Thomas then argues that the union takes place in the person (for in the Hypostatic Union, Christ is one person with two natures). In doing so he gives a brief overview of some technical, philosophical concepts. A nature, as he’s said, is the general essence of a being. The nature of a human is to be a rational animal. All individual humans have this same thing as their essence. A “suppositum” is “the individual subsisting in this nature” [13]. So an individual elephant is a suppositum of the nature of “elephantness”, an individual human is a suppositum of the nature “humaneness”. Next, Aquinas says, “in certain subsisting things we happen to find what does not belong to the notion of the species, viz. accidents and individuating principles” [14]. In other words, each suppositum, or each individual, has certain features/qualities which do not belong to them by virtue of their participation in their particular essence, but rather by virtue of their subsistence as an individual. One human might have brown hair, another black. If the quality of “having brown hair” belonged intrinsically into the essence of humanity, then all humans would have brown hair, and having brown hair would be a part of what made them humans. But that is obviously not the case. A suppositum is just an individual instance of an essence, which contains both the nature, and the “individuating principles”, or things that make it individual, unique, as opposed to identical to every other member of that essence. So Aquinas says: “Hence in such as these the nature and the suppositum really differ; not indeed as if they were wholly separate, but because the suppositum includes the nature, and in addition certain other things outside the notion of the species” [15]. That is significant: the suppositum includes the nature, but the nature does not include the suppositum. If the nature included the suppositum, then every single individual instance of an essence would be exactly identical in every possible way.

Now a “person” is just a suppositum of the essence of “rational or intellectual creatures” [16]. For Aquinas, personhood connotes having intellect and will. A human person is an individual with the essence of humanity. Since this individual, by virtue of human nature, has rational faculties, they are said to be a person. So “what is said of a suppositum is to be applied to a person” [17]. Since, in rational creatures, the person just is the suppositum, then whatever is said of the suppositum is applied to the person. If it is part of the suppositum to have brown hair, then we apply “having brown hair” to that individual person. Thus “whatever adheres to a person is united to it in person, whether it belongs to its nature or not” [18]. Having specifically brown hair does not belong to the human nature, but “having brown hair” can adhere to an individual person, so we say that “having brown hair” is united to a person in the person.

Now in God, there is no actual distinction between nature and suppositum. For God’s suppositum just is His nature; they are identical in Him. But, says Aquinas:

“If there is a thing in which there is nothing outside the species or its nature (as in God), the suppositum and the nature are not really distinct in it, but only in our way of thinking, inasmuch it is called ‘nature’ as it is an essence, and a suppositum as it is subsisting” (emphasis mine) [19].

So although in God the nature and suppositum are not actually distinct, we can refer to them, conceptually, distinctly. The purpose of all this is such: in Christ, the human nature and Divine Nature are united. But “having a human nature” as such does not belong intrinsically to the Divine Nature (otherwise the entire Godhead, in Itself, would be a human being). So the human nature of Christ must be united to him in the person, not in the nature:

“Therefore, inasmuch as the Word has a human nature united to Him, which does not belong to His Divine Nature, it follows that the union took place in the Person of the Word, and not in the nature” [20].

(The reader may now wonder: “but wait, if in God nature and suppositum are identical, how can it be that one Divine Person can be united to a human nature, without thereby having that human nature united to all Three Persons? For if the human nature is united to the Divine Person, and the Person, as suppositum, is identical to the Essence, then mustn’t the human nature be united to the Divine Essence, and thus to all Three Persons? Aquinas discusses how exactly the Three Persons are distinct in Summa Theologiae Part I, Question 39, Article 1. Because of the significance and complexity of the topic of the Trinity, that will not be discussed further here. But Aquinas is adamant to maintain that the “human nature is united to the Word, so that the Word subsists in it, and not so that His Nature receives therefrom any addition or change” (emphasis mine) [21]. The Hypostatic Union of the two natures in One Person is such that the Divine Nature is not in any way altered, changed, or added to by the human nature, for Divine Nature by such is of infinite fullness, so that it cannot receive anything unto it. Rather, the One Person subsists in both the Divine and Human natures.)

Remember that a person is a suppositum, and a suppositum is an individual instance of a particular essence. So in the Hypostatic Union, one Person/suppositum is the individual instance of two distinct essences/natures. Remember that the nature is in the suppositum, such that in Christ there are two natures, one human, one Divine. Christ as a person exists eternally–before the Incarnation as His Divine Nature, but at the Incarnation he “assumes” the human nature which is joined in union with the Divine, although both remain distinct.

A little later on, Aquinas considers the relation between “union” and “assumption”. In the Hypostatic Union, two distinct natures are brought together in union. This union is a “relation” between the two natures. But both natures are not “assumed” by the One Person. For assumption, says St. Thomas, implies “becoming” and the “action” of being united, and is “a taking to oneself from another” [22], and all these involve changes. But the Divine Nature, being immutable, can admit to no change; so it is not the Divine Nature which is “assumed” or “taken on” by the Person, rather it is the human nature which is assumed to the Divine Nature by and in the Person.

“Hence it may be said indifferently that the human nature is united with the Divine, or conversely. But the Divine Nature is not said to be assumed by the human, but conversely, because the human nature is joined to the Divine Personality, so that the Divine Person subsists in human nature” [23].

Now we might ask, how did the Divine Person, with its eternal Divine Nature, assume at some point in history a human nature? Would this not involve a change in the Divine Person, which is intrinsically impossible?

Not so, answers St. Thomas. For in being assumed, the human nature is joined in union to the Divine, and this Union is essentially a relation between the human and Divine natures; and (as he has established elsewhere in the Summa),

“every relation which we consider between God and the creatures 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, it is really” [24].

So the Incarnation does not involve a change in the Divine Person–there is nothing altered in or intrinsically added to the Divine Nature. Rather, the Union of the natures is a relation in the human nature, not the Divine. This might seem like a contradiction, since Aquinas earlier contended that the union takes place not in the nature but in the Person. But there he was referring to the actual “act” of the union, of the coming together, and here he is referring to the relation. The “act” of the union takes place in the Person, but the relation is in the Person subsisting in the human nature.

There is much, much more that the Angelic Doctor says concerning this glorious, profound Mystery of Incarnation and Hypostatic Union. And even what I’ve shared here has just been a brief, surface level summary of each argument. For a fuller treatment/understanding, I highly recommend reading these sections in their entirety in the Summa for yourself. This post is meant merely as a sort of introduction to St. Thomas’s thinking about the Incarnation. Hopefully this was enough to show that there is no strict metaphysical contradiction in the concept of Incarnation, since the Divine Nature remains totally as itself.

In conclusion, we will look at the “fittingness” of Christmas itself. Was it fitting that God should become a man?

There are four objections. First, “since God from all eternity is the very essence of goodness, it was best for Him to be as He had been from all eternity” [25], and, since God has not been with flesh from all eternity, it would be best for Him to remain without flesh. Second, “God and flesh are infinitely apart”, and “it is not fitting to unite things that are infinitely apart”, so “therefore it was not fitting that God should be united to human flesh” [26]. The third is similar to the second, that “it was not fitting that the highest uncreated spirit should assume a body” [27]. And the fourth should be written in full:

“Further, it is not becoming that He Who surpassed the greatest things should be contained in the least, and He upon Whom rests the care of great things should leave them for lesser things. But God–Who takes care of the whole world–the whole universe of things cannot contain. Therefore it would seem unfitting that ‘He should be hid under the frail body of a babe in swathing bands, in comparison with Whom the whole universe is accounted as little; and that this Prince should quit His throne for so long, and transfer the government of the whole world to so frail a body,’ as Volusianus writes to Augustine (Ep. cxxxv)” [28].

And then Aquinas makes his beautiful reply:

On the Contrary, It would seem most fitting that by visible things the invisible things of God should be made known; for to this end was the whole world made . . .” [29].

In short, the very purpose and meaning of the universe is to make known God, and this is done completely, being wholly fulfilled, in the Incarnation at Christmas. Thus Christmas, in effect, is the very purpose and meaning of the universe.

He continues:

“By the mystery of the Incarnation are made known at once the goodness, the wisdom, the justice, and the power or might of God–‘His goodness, for He did not despise the weakness of His own handiwork; His justice, since, on man’s defeat, He caused the tyrant to be overcome by none other than man, and yet He did not snatch men forcibly from death; His wisdom, for He found a suitable discharge for a most heavy debt; His power, or infinite might, for there is nothing greater than for God to become incarnate . . .'” [30].

What wondrous words! I’m reminded of the Chesterton quote: “The riddles of God are more satisfying than the solutions of men.”

“To each thing, that is befitting which belongs to it by reason of its very nature; thus, to reason befits man, since this belongs to him because he is of a rational nature. But the very nature of God is goodness . . . Hence, what belongs to the essence of goodness befits God. But it belongs to the essence of goodness to communicate itself to others, as is plain from Dionysius (Div. Nom. iv). Hence it belongs to the essence of the highest good to communicate itself in the highest manner to the creature, and this is brought about chiefly by ‘His so joining created nature to Himself that one Person is made up of these three–the Word, a soul and flesh,’ as Augustine says (De Trin. xiii). Hence it is manifest that it was fitting that God should become incarnate” [31].

Thus, as I wrote in my previous post, the Incarnation truly is the answer to the silence of God, for the Incarnation is the act of the Highest Good communicating Itself in the highest manner to the creatures, communicating Itself so deeply and profoundly that It would take on flesh and become one of us. So the writer of Hebrews declares:

“The Son is the radiance of God’s glory and the exact representation of his being” (Hebrews 1:3, NIV).

In Christ, in the Son, we see and know the very Nature of who God is.

Briefly, to reply specifically to that fourth objection, Aquinas quotes Augustine (Ep. ad Volusian. cxxxvii):

“‘The Christian doctrine nowhere holds that God was so joined to human flesh as either to desert or lose, or to transfer and as it were, contract within this frail body, the care of governing the universe. This is the thought of men unable to see anything but corporeal things . . . God is great not in mass, but in might. Hence the greatness of His might feels no straits in narrow surroundings. Nor, if the passing word of a man is heard at once by many, and wholly by each, is it incredible that the abiding Word of God should be everywhere at once?” Hence nothing unfitting arises from God being incarnate” [32].

How terrifying and awesome to think, that two thousand years ago a little babe lay in a small manger, in an insignificant, tiny town, born to peasants in a nation subdued to the might of the Roman empire, and that this babe Himself upheld by His own power the whole of the universe, that this babe was the physical incarnation of Existence Itself, Pure Being, Goodness Itself, full of infinite power, knowledge, etc. This is the glory, beauty, and mystery of the Incarnation. This is Christmas.



[1]. Pace, Edward. “Hypostatic Union.” The Catholic Encyclopedia. Vol. 7. New York: Robert Appleton Company, 1910.20 Dec. 2016 <;.

[2]. Aquinas, Thomas. Summa Theologica. 1265-1274. Trans. Fathers of the English Dominican Province. Coyote Canyon Press, 2010. ebook. III, Q. 2, Art. 1.

[3]. Ibid.


[5]. Ibid.

[6]. Ibid.

[7]. Ibid.

[8]. Ibid.

[9]. Ibid.

[10]. Ibid.

[11]. Ibid.

[12]. Ibid.

[13]. Ibid. III, Q. 2, Art. 2.

[14]. Ibid.

[15]. Ibid.

[16]. Ibid.

[17]. Ibid.

[18]. Ibid.

[19]. Ibid.

[20]. Ibid.

[21]. Ibid.

[22]. Ibid. III, Q. 2, Art. 8.

[23]. Ibid.

[24]. Ibid.

[25]. Ibid. III, Q. 1, Art. 1.

[26]. Ibid.

[27]. Ibid.

[28]. Ibid.

[29]. Ibid.

[30]. Ibid.

[31]. Ibid.

[32]. Ibid.

Cover image in the public domain, Photo: Myrabella / Wikimedia Commons, via Wikimedia Commons


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