Every voluntary agent loves himself. God, having none higher than Himself, is not called upon to subordinate His love of Himself to any other love. But angels, or man, while naturally, necessarily, and rightly loving himself, is further called upon to subordinate his love of himself to the love of God. This further call he may fail to comply with: then he sins.

The argument in the text is directed against the Platonists, who ascribed all sinful tendency to the body, and consequently could see no possibility of sin in a pure spirit, or angel, and thus came to reject the Christian dogma of the fall of the angels.

Of God and His Creatures: 3.109