Chapter 20.—Against Eunomius, Saying that the Son of God is the Son, Not of His Nature, But of His Will. Epilogue to What Has Been Said Already.
38. Wherefore the logic of Eunomius, from whom the Eunomian heretics sprang, is ridiculous. For when he could not understand, and would not believe, that the only-begotten Word of God, by which all things were made is the Son of God by nature,—i.e. born of the substance of the Father,—he alleged that He was not the Son of His own nature or substance or essence, but the Son of the will of God; so as to mean to assert that the will by which he begot the Son was something accidental [and optional] to God,—to wit, in that way that we ourselves sometimes will something which before we did not will, as though it was not for these very things that our nature is perceived to be changeable,—a thing which far be it from us to believe of God. For it is written, "Many are the thoughts in the heart of man, but the counsel of the Lord abideth for ever,"  for no other reason except that we may understand or believe that as God is eternal, so is His counsel for eternity, and therefore unchangeable, as He himself is. And what is said of thoughts can most truly be said also of the will: there are many wills in the heart of man, but the will of the Lord abideth for ever. Some, again, to escape saying that the only-begotten Word is the Son of the counsel or will of God, have affirmed the same Word to be the counsel or will itself of the Father. But it is better in my judgment to say counsel of counsel, and will of will, as substance of substance, wisdom of wisdom, that we may not be led into that absurdity, which we have refuted already, and say that the Son makes the Father wise or willing, if the Father has not in His own substance either counsel or will. It was certainly a sharp answer that somebody gave to the heretic, who most subtly asked him whether God begat the Son willingly or unwillingly, in order that if he said unwillingly, it would follow most absurdly that God was miserable; but if willingly, he would forthwith infer, as though by an invincible reason, that at which he was aiming, viz. that He was the Son, not of His nature, but of His will. But that other, with great wakefulness, demanded of him in turn, whether God the Father was God willingly or unwillingly; in order that if he answered unwillingly, that misery would follow, which to believe of God is sheer madness; and if he said willingly, it would be replied to him, Then He is God too by His own will, not by His nature. What remained, then, except that he should hold his peace, and discern that he was himself bound by his own question in an insoluble bond? But if any person in the Trinity is also to be specially called the will of God, this name, like love, is better suited to the Holy Spirit; for what else is love, except will?
39. I see that my argument in this book respecting the Holy Spirit, according to the Holy Scripture, is quite enough for faithful men who know already that the Holy Spirit is God, and not of another substance, nor less than the Father and the Son,—as we have shown to be true in the former books, according to the same Scriptures. We have reasoned also from the creature which God made, and, as far as we could, have warned those who demand a reason on such subjects to behold and understand His invisible things, so far as they could, by those things which are made  and especially by the rational or intellectual creature which is made after the image of God; through which glass, so to say, they might discern as far as they could, if they could, the Trinity which is God, in our own memory, understanding, will. Which three things, if any one intelligently regards as by nature divinely appointed in his own mind, and remembers by memory, contemplates by understanding, embraces by love, how great a thing that is in the mind, whereby even the eternal and unchangeable nature can be recollected, beheld, desired, doubtless that man finds an image of that highest Trinity. And he ought to refer the whole of his life to the remembering, seeing, loving that highest Trinity, in order that he may recollect, contemplate, be delighted by it. But I have warned him, so far as seemed sufficient, that he must not so compare this image thus wrought by that Trinity, and by his own fault changed for the worse, to that same Trinity as to think it in all points like to it, but rather that he should discern in that likeness, of whatever sort it be, a great unlikeness also.