The most common argument raised against the existence of God is the so-called problem of evil. How can an all-good, all-powerful God allow evil to exist? However, neither the atheist nor the relativist can raise the problem of evil since when you admit the existence of evil you are admitting the existence of objective moral values. The atheist cannot adequately ground objective morality and the relativist assumes that morality is relative.
In other words, the problem of evil cannot even be raised without assuming an objective standard of moral goodness. By “objective” I mean independent of what people think or perceive. Complaining about evil assumes that evil is a real thing that it is objectively wrong, otherwise we could simply dismiss the atheist or relativist by saying “that’s just evil for you.”
So where does this objective standard of morality come from? The only suitable grounding for objective morality is an objective moral law-giver: God. Ironically then, the existence of evil can be turned into an argument for the existence of God:
1. If God does not exist, objective moral values do not exist.
2. Evil exists.
3. Therefore, objective moral values exist.
4. Therefore, God exists.
This argument is logically valid. The skeptic concedes premise two by raising the problem of evil in the first place. Therefore, the argument hinges on premise one. However, in reflecting on premise one it seems clear that if there is no God then there is no objective grounding for moral principles which apply to all people, in all places, at all times. Morality would be relegated to cultural conventions or individual ethical subjectivism. William Lane Craig sums it up this way:
Although at a superficial level suffering calls into question God’s existence, at a deeper level suffering actually proves God’s existence. For apart from God, suffering is not really bad. If the atheist believes that suffering is bad or ought not to be, then he’s making moral judgments that are possible only if God exists.
In short, when the atheist or relativist objects to the problem of evil he implicitly admits to an objective standard of morality which his own worldview cannot account for, but which makes perfect sense within the Christian worldview. In other words, in order to complain about evil and raise the objection in the first place, atheists, skeptics, and relativists must borrow from Christian moral capital and the Christian worldview.
 William Lane Craig, Reasonable Faith: Christian Truth and Apologetics, 3rd ed. (Wheaton: Crossway, 2008), 173.
 William Lane Craig, Hard Questions, Real Answers (Wheaton: Crossway, 2003), 107.
 William Lane Craig, On Guard: Defending Your Faith with Reason and Persuasion (Colorado Springs: David Cook, 2010), 162.