The doctrine of God’s omnibenevolence (God’s complete goodness) raises the problem of evil.
The classic formulation of this problem is the apparent incompatibility of the three propositions: 1) God is wholly God, 2) God is all-powerful, and 3) evil exists. If God is wholly good, he would want to prevent gratuitous evil; and if he were all-powerful, he would be able to prevent gratuitous evil (evil with no justifying cause). Yet evil still exists. Therefore, the Christian idea of an all-powerful, omnibenevolent God appears to be incoherent in a world in which evil exists.
On a philosophical level, this problem is fairly simple to explain. The objector has failed to make explicit a proposition that is unprovable, yet essential to the success of the argument: that is, that there is at least one instance of evil that is gratuitous. In other words, for any instance of evil that appears to be gratuitous, God might have a reason leading to a greater good of which we are currently unaware. Yet to have this knowledge, the objector must have omniscience.
The person who wishes to demonstrate the incoherence of the idea of God based on the existence of evil also faces the problem how where he or she got the idea of evil in the first place. Objectors to Christian theism would like to say that the existence of evil makes the concept of God incoherent. Upon further analysis, however, it becomes apparent that if God did not exist, the concept of evil itself would become incoherent. Therefore, by making his or her argument from evil, the atheist or skeptic has secretly imported some idea of absolute good.
Without God, you can’t have absolute good. And without absolute good, you have no right to speak of evil.