On the Christian Faith (De fide)

The likeness of the Son to the Father being proved, it is not hard to prove the Son's eternity, though, indeed, this may be established on the authority of the Prophet Isaiah and St. John the Evangelist, by which authority the heretical leaders are shown to be refuted.

54. It is plain, therefore, that the Son is not unlike the Father, and so we may confess the more readily that He is also eternal, seeing that He Who is like the Eternal must needs be eternal. But if we say that the Father is eternal, and yet deny this of the Son, we say that the Son is unlike the Father, for the temporal differs from the eternal. The Prophet proclaims Him eternal, and the Apostle proclaims Him eternal; the Testaments, Old and New alike, are full of witness to the Son's eternity.

55. Let us take them, then, in their order. In the Old Testament— to cite one out of a multitude of testimonies— it is written: “Before Me has there been no other God, and after Me shall there be none.” I will not comment on this place, but ask you straight: “Who speaks these words—the Father or the Son?” Whichever of the two you say, you will find yourself convinced, or, if a believer, instructed. Who, then, speaks these words, the Father or the Son? If it is the Son, He says, “Before Me has there been no other God;” if the Father, He says, “After Me shall there be none.” The One has none before Him, the Other none that comes after; as the Father is known in the Son, so also is the Son known in the Father, for whenever you speak of the Father, you speak also by implication of His Son, seeing that none is his own father; and when you name the Son, you do also acknowledge His Father, inasmuch as none can be his own son. And so neither can the Son exist without the Father, nor the Father without the Son. The Father, therefore, is eternal, and the Son also eternal.

56. “In the beginning was the Word, and the Word was with God, and the Word was God. The same was in the beginning with God.” “Was,” mark you, “with God.” “Was”— see, we have “was” four times over. Where did the blasphemer find it written that He “was not.” Again, John, in another passage— in his Epistle— speaks of “That which was in the beginning.” The extension of the “was” is infinite. Conceive any length of time you will, yet still the Son “was.”

57. Now in this short passage our fisherman has barred the way of all heresy. For that which was “in the beginning” is not comprehended in time, is not preceded by any beginning. Let Arius, therefore, hold his peace. Moreover, that which was “with God” is not confounded and mingled with Him, but is distinguished by the perfection unblemished which it has as the Word abiding with God; and so let Sabellius keep silence. And “the Word was God.” This Word, therefore, consists not in uttered speech, but in the designation of celestial excellence, so that Photinus' teaching is refuted. Furthermore, by the fact that in the beginning He was with God is proven the indivisible unity of eternal Godhead in Father and Son, to the shame and confusion of Eunomius. Lastly, seeing that all things are said to have been made by Him, He is plainly shown to be author of the Old and of the New Testament alike; so that the Manichæan can find no ground for his assaults. Thus has the good fisherman caught them all in one net, to make them powerless to deceive, albeit unprofitable fish to take.