Book Four — Salvation, Chapter 8 — Summa Contra Gentiles
[1] Since, however, truth cannot be truth’s contrary, it is obvious that the points of Scriptural truth introduced by the Arians to confirm their error cannot be helpful to their teaching. For, since it was shown from divine Scripture that the essence and divine nature of the Father and Son are numerically identical, and according to this each is called true God, it must be that the Father and Son cannot be two gods, but one God. For, if there were many gods, a necessary consequence would be the partition in each of the essence of divinity, just as in two men the humanity differs in number from one to the other; and the more so because the divine nature is not one thing and God Himself another. This was shown above. From this it follows necessarily that, since there exists one divine nature in the Father and the Son, the Father and the Son are one God. Therefore, although we confess that the Father is God and the Son God, we are not withdrawing from the teaching which sets down that there is one only God, which we established both by reasonings and by authorities in Book I. Hence, although there is one only true God, we confess that this is predicated of the Father and of the Son.
[2] When our Lord, therefore, speaking to the Father, says “that they may know You the only true God,” it is not so to be understood that the Father alone is true God, as though the Son is not true God (the contrary is proved clearly by Scriptural testimony); but it must be understood that the one sole true deity belongs to the Father, in such wise, nonetheless, that the Son is not excluded therefrom. Hence, it is significant that our Lord does not say: “that they may know the one only true God,” as though He alone be God, but said: “that they may know You,” and added “the only true God” to show that the Father, whose Son He insisted He was, is the God in whom one finds that only true divinity. And because a true son must be of the same nature as his father, it follows that the only true divinity belongs to the Son, rather than that the Son is excluded from it. Wherefore John, also, at the end of his first canonical Epistle (5:20)—expounding, as it were, these words of our Lord—attributes to the true Son each of the things which our Lord here says of the Father; namely, that He is true God and that in Him is eternal life. John says (5:20): “That we may know the true God, and may be in His true Son. He is the true God and life eternal.”
If the Son had nevertheless confessed that the Father alone is true God, one would not for this reason need to understand that the Son is excluded from true divinity. For, since the Father and Son are one God, as was shown, whatever is said of the Father by reason of divinity is the same as if it were said of the Son, and conversely. For, by reason of the fact that our Lord says: “No one knows the Son but the Father: neither does any one know the Father but the Son” (Mat. 11:27), it is not understood that the Father is excluded from knowledge of Himself, or that the Son is.
[3] It is also clear from this that the true divinity of the Son is not excluded by the words of the Apostle: “Which in His times He shall show who is the Blessed and only Mighty, the King of kings, and Lord of lords.” In these words the Father is not named, but that which is common to the Father and the Son. That the Son is the King of kings and Lord of lords is manifestly shown in the Apocalypse (19:13), which says: “He was clothed with a garment sprinkled with blood; and His name is called THE WORD OF GOD”; and adds below: “And He has on His garment and on His thigh written: KING OF KINGS, AND LORD OF LORDS” (19:i6). Nor is the Son excluded from that which is added: “Who only has immortality,” since He also bestows immortality on those who believe in Him. Thus, John (11: 26) says: “Who believes in Me shall not die for ever.” But what is added,” “Whom no man has seen, nor can see,” certainly is also suitable to the Son, since our Lord says: “No one knows the Son but the Father” (Mat. 11:27). To this it is not an objection that He appeared visibly, for this was according to the flesh. However, He is invisible in His deity just as the Father is; wherefore the Apostle says in the same Epistle (1 Tim. 3:16): “Evidently great is the mystery of godliness, which was manifested in the flesh.” Nor are we forced to understand these sayings of the Father alone because it is said that there must be one who shows and another who is shown. The Son also shows Himself, for He says: “He that loves Me shall be loved of My Father: and I will love him, and will manifest Myself to him” (John 14:21.). Accordingly, we also say to Him: “Shew us your face, and we shall be saved” (Ps. 79:4).
[4] But how the saying of our Lord, “The Father is greater than I” must be understood we are taught by the”Apostle. Since “greater” is referred to ‘lesser,” one must understand that this is said of the Son so far as He is lessened. Now, the Apostle shows that He is lessened by taking on the servile form—in such wise, however, that in the divine form He exists the equal of God the Father, for he says: “Who being in the form of God, thought it not robbery to be equal with God: but emptied Himself, taking the form of a servant” (Phil. 2:6-7). Nor is it wondrous if for this reason the Father be said to be greater than He, since He was even made lesser than the angels; the Apostle says: “We see Jesus, who was made a little lesser than the angels, for the suffering of death, crowned with glory and honour” (Heb. 2:9).