The Summa Contra Gentiles on the Incarnation (a look at some early heresies)

In the Summa Contra Gentiles, Thomas begins his treatment of the Incarnation after his treatment of the Triune nature of God.  In that previous part, he had already said a good deal about the second Person of the Trinity, and likewise, we began his treatment with heresies and their refutations.  This approach is obviously different than the more strictly systematic approach of the Summa Theologica, but it is a very effective at demonstrating the truths of the faith.

We will, of course, see much overlap in the teachings as presented in the two great “summas” of the Angelic Doctor.  The order of teaching is different, to be sure, but much of the content is certainly the same; that of the truth of the Incarnation, which is and its proper understanding, at least insofar as it is understandable by the limited human intellect, for “it follows that toward faith in this particular marvel all other miracles are ordered, since ‘that which is greatest in any genus seems to be the cause of the others.’”1

We will first examine the error of Photinus, first treated in Question 28.  Thomas tells us that “according to this position, God would not have assumed flesh to become man; rather, an earthly man would have become God. Thus, the saying of John (1:14) would not be true: ‘The Word was made flesh’; on the contrary, flesh would have been made the Word.”2

Thomas treats this in a different way in Question 16 of the third part of the Summa Theologica. “Properly understood, this participle “made” attaches making to man with relation to God, as the term of the making… this proposition is false, because, when it is said, “Man was made God,” “man” has a personal suppositum: because, to be God is not verified of the Man in His human nature, but in His suppositum.”3

Why can we not say that man became God? A wall that has always existed can be made white, but white cannot be “made a wall,” for white must exist in something.  Now, we do not predicate Christ’s humanity as a metaphysical accident, but we do affirm that its existence is in the Person of Christ.  For man to be made God, we would have to adhere to the heresy of Nestorius, for a man cannot exist without a person existing, and so either this person would have to have been annihilated when He became Christ, or else there would be two persons in Christ.

“Let one consider the matter earnestly and he sees that this Nestorian opinion on the Incarnation differs very little from that of Photinus…Of course, on the eternal generation of the Word they differed greatly: Nestorius confessed it, but Photinus denied it completely.”4 We see how extreme and opposite errors tend to meet in some of their conclusions.

We now turn to this heresy, also addressed by St. Thomas. Thomas’ treatment against the error of Nestorius is among the longest of his refutations of error in Book IV of the Summa Contra Gentiles. The position of this error is described as follows: “They said that the human soul and the true human body came together in Christ by a natural union to constitute one man of the same species and nature with other men, and that in this man God dwelt as in His temple, namely, by grace, just as in other holy men.”5

This, however, must result in a complete denial of the Incarnation: Christ was made man cannot be true if understood in this way.  For God has dwelled in many men in the way spoken of by Nestorius, and even if a difference in “degree” of this indwelling is claimed, it cannot be a true difference in kind.

Understood in this way, Christ would merely be the most favored of all humans.  He would be similar to Mary in this way, only greater.  This is obviously not in keeping with the faith. “For, in that position, the Word of God was united to that man only through an indwelling by grace, on which a union of wills follows. But the indwelling of God’s Word in a man is not for God’s Word to be made flesh.”6

Likewise, in this way of understanding the Incarnation, if “our understanding of the Incarnation of the Word is this alone—the Word of God dwelt most fully in that man—we will have to say that the Holy Spirit was incarnate also.”7 Obviously, this position is untenable.

Thomas, in book IV of the Summa Contra Gentiles, will often use the authority of the Church and her Councils.  This is perfectly in line with the purpose of Book IV, which, unlike the first three books, makes no claim to be of things demonstrable from human reason alone apart from revelation.  “Now, it was proved that the body of that man is the body of the natural Son of God, that is, of the Word of God. So it becomes us to say that the Blessed Virgin is ‘the Mother of the Word of God,’ and even ‘of God’.”8

Objection to the title “Mother of God” arose in the fifth century, due to confusion concerning the mystery of the incarnation. Nestorius, stated that Mary gave birth to Jesus Christ, a regular human person. As such, Mary is not “Mother of God,” but simply the “Mother of Christ” or even the “Mother of Christ’s humanity.”  Our protestant brethren would do well to contemplate the wisdom of the ancient Church councils and of Thomas Aquinas’ masterful expositions of such topics, as presented in the work this short essay presents.

We now turn a position, also heretical and indeed contrary to even reason alone, that was taken in opposition to the previous heresy of Nestorius. “Eutyches…says there is one nature, also. He says that, although before the union there were two distinct natures, the divine and human, they came together, nevertheless, in the union into one nature.”9 Thomas demonstrates that this is both repugnant to Scripture and to reason.

“If…the human nature and the divine were two before the union, but from those in the union one nature was breathed together, this should take place in one of the ways in which it is natural that one comes to be from many.”10 In the next few articles, Thomas goes through the various ways this can be said to take place in nature, and demonstrates that all are untenable. The most important of these is the idea of a “mixture,” which can result in a single nature only through the destruction of both of the joined natures. Robert Sokolowski demonstrates why the pagans could never understand this idea of an Incarnate God:

“The reason pagans could not conceive of anything like the incarnation is that their gods are part of this world, and the union of any two natures in this world is bound to be unnatural, because of the otherness that lets one thing be itself only by not being the other…The Christological heresies are a reflection of tendencies to make pagan the Christian sense of the divine.”11

Thomas, of course, understood that God cannot even be understood to be in a genus.  He is not differentiated, therefore, even within the genus of being.  It is this infinite difference that makes possible the relation between man and God when God takes on a human nature.  Part of the definition and essence of a rock is that it is not a tree, not Socrates, etc. There is no mixture, and no destruction, for there is no relatedness between God and anything created that would cause one to be defined by “not being the other.” Only in this way, rejecting our common understanding of how things combine within contingent being, can a true understanding of the Incarnation ever take place.

I am of the opinion that Thomas’ understanding of the difference between essence and existence, as well as his understanding of God’s essence as “to exist,” is the most outstanding breakthrough in understanding of the mysteries of God in the last 2000 years.


  1. SCG Bk IV, 27,1
  2. Ibid, 28, 4
  3. ST. III, 16, art. 7
  4. SCG Bk IV, 34, 31
  5. Ibid, 34, 2
  6. Ibid, 34, 3
  7. Ibid, 34, 23
  8. Ibid, 34, 15
  9. Ibid, 35, 2
  10. Ibid, 35, 6
  11. Sokolowski, The God of Faith and Reason, p. 36

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