The Enchiridion

These are the great works of the Lord, sought out according to all His pleasure, and so wisely sought out, that when the intelligent creation, both angelic and human, sinned, doing not His will but their own, He used the very will of the creature which was working in opposition to the Creator's will as an instrument for carrying out His will, the supremely Good thus turning to good account even what is evil, to the condemnation of those whom in His justice He has predestined to punishment, and to the salvation of those whom in His mercy He has predestined to grace.

For, as far as relates to their own consciousness, these creatures did what God wished not to be done: but in view of God's omnipotence, they could in no wise effect their purpose. For in the very fact that they acted in opposition to His will, His will concerning them was fulfilled. And hence it is that “the works of the Lord are great, sought out according to all His pleasure,” because in a way unspeakably strange and wonderful, even what is done in opposition to His will does not defeat His will. For it would not be done did He not permit it (and of course His permission is not unwilling, but willing); nor would a Good Being permit evil to be done only that in His omnipotence He can turn evil into good.