Summa contra Gentiles1.39 Chapter XXXIX. That no evil can be in God

Hence it is manifestly apparent that evil cannot be in God. For being and goodness and all essential predicates have nothing besides themselves added to them, although that which is or the good may have something besides being or goodness: since nothing hinders the subject of one perfection being the subject of another besides; thus that which is a body may be white and sweet: while every nature is confined within the bounds of its essence, so that it admits of nothing extraneous within itself. Now God is goodness and not merely good, as we have proved above. Therefore nothing that is not goodness can be in Him: and consequently evil can nowise be in Him.

Moreover. As long as a thing remains, that which is contrary to its essence is altogether incompatible with that thing: thus irrationality or insensibility is incompatible with man unless he cease to be man. Now the divine essence is goodness itself, as we have proved. Therefore evil which is contrary to good can have no place in God unless He cease to be God: which is impossible, since He is eternal, as was proved above.

Again. Since God is His own being, nothing can be said of Him by participation, as is clear from the argument given above. If, then, evil were predicated of Him, it would be a predicate not by participation, but by essence. But evil cannot be predicated of any thing in such a way as to be the essence of that thing: for it would lack being, which is a good, as we have shown above: and in evil there can be no extraneous admixture, as neither can there be in goodness. Therefore evil cannot be predicated of God.

Again. Evil is opposed to good. Now the notion of good consists in perfection: and therefore the notion of evil consists in imperfection. Now defect or imperfection cannot be in God, since He is universally perfect, as shown above. Therefore evil cannot be in God.

Further. A thing is perfect according as it is in act. Therefore it will be imperfect according as it is deficient in act. Therefore evil is either privation or includes privation. Now the subject of privation is a potentiality: and this cannot be in God, and consequently neither can evil.

Moreover. If good is what is desired by all, it follows that evil as such is shunned by every nature. Now that which is in a thing against the mode of its natural appetite is violent and unnatural. Therefore evil in a thing is violent and unnatural in so far as it is an evil to that thing, although in composite things it may be natural thereto in respect of some part. But God is not composite, nor can anything be violent or unnatural in Him, as shown above. Therefore evil cannot be in God.

This is moreover confirmed by Holy Writ. For it is written in the canonical epistle of John: God is light, and in Him there is no darkness; and (Job xxxiv. 10): Far from God be wickedness, and iniquity from the Almighty.