Commentaria in Summam contra Gentiles, Liber IV

The third error, more irrational than the others, was that of Apollinaris, who said that something of the Word of God was converted into flesh, agreeing in this with the aforesaid, that the body of Christ was not assumed from the Virgin. Moreover, he took the occasion for this from what is said: The Word was made flesh: which he interpreted as meaning, The Word was converted into flesh: just as when it is said, When the ruler of the feast had tasted the water made wine, it is understood as meaning the water converted into wine.

I. But against this error it is argued, first. Everything that is converted into another is changed into it. God is utterly immutable. Therefore etc.

Secondly. The Word of God, since He is God, is simple. Therefore, if something of the Word was converted into flesh, it is necessary that the whole Word was converted. Therefore, after the Incarnation the Word of God wholly is not: because that which is converted into another ceases to be what it previously was. But this is impossible: both because the Word of God is eternal; and because Christ is called the Word after the Incarnation, as is clear in Apoc. 19. Therefore etc.

Thirdly. The Word of God, since He is God, does not agree either in genus or in matter with anything else whatsoever: since God is neither in a genus nor has matter. Therefore etc. — The consequence is proved. Because for those things that do not communicate in matter or in one genus, it is impossible for a conversion into one another to occur: as is clear concerning a line and whiteness, an elementary and a heavenly body, and an incorporeal substance.

Fourthly. It follows that in Christ there was not true flesh, nor anything else of this kind: and thus He will not be true man. — The consequence is proved. Because it belongs to the notion of flesh, bones, and the like, that they be from determinate matter.

II. But to the adduced authority it is said that the Word is said to have been made flesh because He assumed flesh, so that He might converse with men and appear visible to them. Hence it is added, And dwelt etc.; and in Baruch it is said that upon earth etc.

It must be observed, from the teaching of St. Thomas on John¹, that the Evangelist said, The Word was made flesh, and not, The Word assumed flesh, in order to show that the union of the Word to the flesh was such that it truly made God man and man God, or that God was man: for something is made into this in order that it may be, just as that which has been made white is white. So, that which has been made man is man.