Augustine's approach was not just brilliant; it was practical. His insight is intellectually credible and emotionally satisfying in that it gives hope and offers meaning to the Christian trying to make sense out of life in a fallen world.
Two Aspects of the Problem
The problem of evil can be phrased in several ways. One approach addresses the origin of evil, prompting the syllogism (a series of statements that form a reasoned argument): 1) God created all things; 2) evil is a thing; 3) therefore, God created evil. If the first two premises are true, the conclusion is inescapable.
This formulation, if sustained, is devastating for Christianity; God would not be good if He actively created evil.
Augustine's solution has not been satisfying to some.
The Substance of the Objection
Isn't it possible that God could have created man immutable in his goodness, yet still have the opportunity to freely choose in other areas? Won't man have immutable goodness in heaven? And will he not also have freedom to choose among certain options? Why not here on earth? Couldn't God construct man's nature such that evil simply was not an option?
Overcoming the Objection
God could have created such a world. Freedom in the larger sense (the ability to make choices) does not require freedom in the narrow sense (the ability to make moral choices).
They miss the big picture, though: God would not have accomplished a second purpose. He not only wanted free creatures; He also wanted plenitude, that is, the greatest good possible. Plenitude--the highest good, the best of all possible worlds--requires more than just general freedom; it requires moral freedom, and that necessarily entails the possibility of evil.
Since all that God made is good, even those things which appear evil only appear that way because of a limited context or perspective. When viewed as a whole, that which appears to be evil ultimately contributes to the greater good.
For example, certain virtues couldn't exist without evil: courage, mercy, forgiveness, patience, the giving of comfort, heroism, perseverance, faithfulness, self-control, long-suffering, submission and obedience, to name a few. These are not virtues in the abstract, but elements of character that can only be had by moral souls. Just as evil is a result of acts of will, so is virtue. Acts of moral choice accomplish both.
The Best of All Worlds
A world that had never been touched by evil would be a good place, but it wouldn't be the best place possible. The best of all worlds would be a place where evil facilitated the development of virtues that are only able to exist where evil flourishes for a time. This would produce a world populated by souls that were refined by overcoming evil with good. The evil is momentary. The good that results is eternal.
What good comes out of a drive-by killing, someone might ask, or the death of a teenager through overdose, or a daughter's rape, or child abuse? The answer is that a commensurate good doesn't always come perceptibly out of those individual situations, though God is certainly capable of redeeming any tragedy. Rather, the greater good results from having a world in which there is moral freedom, and moral freedom makes moral tragedies like these possible.
A Heavenly Twist
This observation reveals an interesting twist in this problem. If morality freely chosen can only happen in a world where evil is possible, then heaven will be a place where there will be no moral growth, where moral choices will not be possible because all the inhabitants of heaven will be immutably good. Growth of the soul only possible and available to inhabitants of a fallen world.
Two Scriptural observations lend credibility to this view. First, in recounting the great heroes of faith, the writer of Hebrews mentions that some were rescued by faith, but others endured by faith "...in order that they might obtain a better resurrection." (Heb. 11:35) Second, St. Paul tells St. Timothy that "...godliness is profitable for all things, since it holds promise for the present life and also for the life to come." (1 Tim. 4:8)
Both of these verses indicate that conditions in this life affect conditions in the next. Bearing up under evil in this life improves our resurrection in the next. Godliness in this life brings profit in the next. These benefits are not available after this life or there would be little urgency to grow now; all eternity would be left in which to catch up.
A deeper, more profound good results when virtue is won by free, moral souls struggling with evil, rather than simply granted to them as an element of their constitution.
Augustine knew that evil was real. Independent evidence (natural theology) was enough to convince him that God existed and that everything He created would be good. Evil, then, must be something real, but not a "thing" in the conventional sense. Evil is not a created thing, but spoiled goodness made possible by the free moral agency of rational creatures. Evil is not something present, but something missing, a privation.
The challenge that God could have created a world of free-will creatures immutable in their goodness is answered by the notion of plenitude, the greatest good. The possibility of evil also makes a greater good possible. God made a world in which true moral decision-making and development of virtues is possible in humans, manifest by persons whose character is formed through growth and struggle.
There's a sound reason why God has allowed man the freedom to choose evil. It doesn't conflict with His goodness. God is neither the author of evil, nor its helpless victim. Rather, precisely because of His goodness He chooses to co-exist with evil for a time, that His goodness may be all the more manifest in those who overcome it by freely choosing to do good and avoid evil.
Romans 8:28: "And we know that to them that love God, all things work together unto good, to such as, according to his purpose, are called to be saints."