City of God

It is with reference to the nature, then, and not to the wickedness of the devil, that we are to understand these words, “This is the beginning of God's handiwork;” for, without doubt, wickedness can be a flaw or vice only where the nature previously was not vitiated. Vice, too, is so contrary to nature, that it cannot but damage it. And therefore departure from God would be no vice, unless in a nature whose property it was to abide with God. So that even the wicked will is a strong proof of the goodness of the nature.

But God, as He is the supremely good Creator of good natures, so is He of evil wills the most just Ruler; so that, while they make an ill use of good natures, He makes a good use even of evil wills. Accordingly, He caused the devil (good by God's creation, wicked by his own will) to be cast down from his high position, and to become the mockery of His angels—that is, He caused his temptations to benefit those whom he wishes to injure by them. And because God, when He created him, was certainly not ignorant of his future malignity, and foresaw the good which He Himself would bring out of his evil, therefore says the psalm, “This leviathan whom You have made to be a sport therein,” that we may see that, even while God in His goodness created him good, He yet had already foreseen and arranged how He would make use of him when he became wicked.