City of God, Book 11, section 17
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 Thou hast 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.
For God would never have created any, I do not say angel, but even man, whose future wickedness He foreknew, unless He had equally known to what uses in behalf of the good He could turn him, thus embellishing the course fo the ages, as it were an exquisite poem set off with antitheses.
Augustine speaks of God’s creating wills that He knew would turn to evil (specifically Satan), and planning in advance all of the good He would bring about because of those wills turned evil. The description of the devil “Good by God’s creation, wicked by his own will,” implies a belief in Augustine that evil in the world is a result of wills, which God created with the full knowledge that they would turn to evil but beyond that with the knowledge of how God would turn that evil to good. He is the just Ruler of unjust wills.