"But neither to the good angels do these things, except as far as God commands, nor do the evil ones do them wrongfully, except as far as He righteously permits. For the malignity of the wicked one makes his own will wrongful; but the power to do so, he receives rightfully, whether for his own punishment, or, in the case of others, for the punishment of the wicked, or for the praise of the good." Augustine, On the Trinity, in Nicene and Post-Nicene Fathers of the Christian Church, vol. 3, ed. Philip Schaff (Grand Rapids, MI: Wm. B. Eerdmans Publishing Company, 1997), pg 61, 3.8.13.
If God punishes willing sin with sin and purifies the saved through suffering, God is then just in empowering and maintaining the conditions whereby sin can exist so that willing sin can be punished and the saved purified. In both the case of the sinner and the righteous something good is happening, because the wicked are being punished by sinning and the righteous are purified by being sinned against.
God’s justice requires that he only empower or maintain that which is good and empowering sinners to sin is their punishment and is therefore just. By empowering we (Augustine and I) don’t mean direct action but energizing or maintaining the conditions whereby Satan or a wicked person can act. God establishes the good by grace (unmerited Divine intervention for good) and allows evil by withdrawing grace. God softens the heart with grace and justly hardens the heart by withdrawing grace.
Textual Defense:
Sinning is a punishment for willing sin:
Psalm 81:11-12—But my people did not listen to my voice; Israel would not submit to me. So I gave them over to their stubborn hearts, to follow their own counsels.
All things work for the good of the righteous:
Romans 8:28—And we know that for those who love God all things work together for good, for those who are called according to his purpose.
God sovereignly controls wicked actions for the good:
Genesis 50:20—As for you, you meant evil against me, but God meant it for good, to bring it about that many people should be kept alive, as they are today.
Strength/Weakness:
The depths of the “mystery of lawlessness” (2 Thess. 2:7) are greater than this. The statement explains God’s justice after the Fall, but not before. Yet as a rational defense for God’s justice in current conditions, it should comfort the saints and befuddle the enemies of God.