The following argument was quoted to me by a classmate, attributed to Epicurus the philosopher:
"Is God willing to prevent evil, but not able? Then he is not omnipotent.
"Is he able, but not willing? Then he is malevolent.
"Is he both able and willing? Then whence cometh evil?
"Is he neither able nor willing? Then why call him God?"
It is, on the surface, a formidable argument, as it appears to make a powerful case by contradiction. However, I contend that his case was weakened by several implicit assumptions.
"Is God willing to prevent evil, but not able? Then he is not omnipotent." Granted
"Is he able, but not willing? Then he is malevolent." This is where Epicurus makes his first implicit assumption. If God is able to prevent evil, does that necessarily mean that the entire responsibility for evil lies with him? Consider also, that the only way God could prevent evil would be by the removal of the free will of human kind. We are not free unless we are free to choose wrongly. Perhaps God is not malevolent, but values freedom above the complete absence of evil.
"Is he both able and willing? Then whence cometh evil?" Here again, the point must be made that mere capability does not of necessity produce responsibility. Also, missing throughout this argument is a definition of "evil." If evil merely means what is harmful to humanity, then by Epicurus's own reductionism it must mean what is good for individual humans. (Never mind that reductionism makes it nearly impossible to define good or evil at all.) Since what is beneficial to one may be harmful to another, evil by definition could not be prevented, even by omnipotence. If we define evil as being that which is against God, then he did not produce it. Evil stems from the free will of mankind, and to remove evil as a choice would be to remove that free will. Good would then no longer be good, no longer normative, but merely descriptive. The entire power of the concept of good and evil lies in the choice. Evil exists because the choice exists, and because some have chosen evil. The responsibility lies with those who have made that choice. (All of us have, at some point.)
"Is he neither able nor willing? Then why call him God?" The singular point as it stated is granted, but since we have broken the argument on two of the other possible predicates, there is no longer much to consider here.
C.S. Lewis once made the point that evil cannot exist in and of itself, but only as a perversion, contention to, or dark reflection of good. Evil is not a creation of God, but a perversion of his creation by creatures of free will. Evil is a problem with humanity, not with God. God does not allow evil to exist because he does not care or because it pleases him. Rather, we have created evil of our own devices, and ensnared ourselves in it. God is our only hope of escape, not our ultimate scapegoat. The point should also be observed that if evil is dependent on good, it can never ultimately defeat good. With this consideration, it seems perfectly reasonable that evil may be permitted to exist for a time, that its final defeat may be the greater.