Concerning the Nature of Good, Against the Manichaeans

But to the most excellent creatures, that is, to rational spirits, God has offered this, that if they will not they cannot be corrupted; that is, if they should maintain obedience under the Lord their God, so should they adhere to his incorruptible beauty; but if they do not will to maintain obedience, since willingly they are corrupted in sins, unwillingly they shall be corrupted in punishment, since God is such a good that it is well for no one who deserts Him, and among the things made by God the rational nature is so great a good, that there is no good by which it may be blessed except God. Sinners, therefore, are ordained to punishment; which ordination is punishment for the reason that it is not conformable to their nature, but it is justice because it is conformable to their fault.