(CG 1/038). That God is His own Goodness (Book 1: Of God As He Is In Himself) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

ESSENTIAL being, and essential goodness, and all other things that bear the name of 'essential,' contain no admixture of any foreign element; although a thing that is good may contain something else besides being and goodness, for there is nothing to prevent the subject of one perfection being the subject also of another. Everything is contained within the bounds of its essential idea in such sort as to render it incapable of containing within itself any foreign element. But God is goodness, not merely good. There cannot therefore be in Him anything that is not goodness, and so evil cannot be in Him at all.

3. As God is His own being, nothing can be said of God that signifies participation. If therefore evil could be predicated of Him, the predication would not signify participation, but essence. Now evil cannot be predicated of any being so as to be the essence of any: for to an essentially evil thing there would be wanting being, since being is good. There cannot be any extraneous admixture in evil, as such, any more than in goodness. Evil therefore cannot be predicated of God.

5. A thing is perfect in so far as it is in actuality: therefore it will be imperfect inasmuch as it is failing in actuality. Evil therefore is either a privation, or includes a privation, or is nothing. But the subject of privation is potentiality; and that cannot be in God: therefore neither can evil.

This truth also Holy Scripture confirms, saying: God is light, and there is no darkness in Him, (I John i, 5) Far from God impiety, and iniquity from the Almighty (Job xxxiv, 10).