I attend a study every other Friday at a friend's house where we discuss, among other things, whether there is a rational basis for belief in God and Christ. The chapter in the book we read for this week covered the question of why there is evil in the world. This is a question that theologians have wrestled with for centuries.
The existence of evil is a prominent criticism of Christianity by non-religious persons. In short, the argument goes something like this: If God created all things, and evil exists, then God created evil. Therefore, God is either one of two things - 1) bad for allowing evil, or 2) not all powerful because He doesn't stop bad things from happening. Neither option is a good and some, in turn, find that they cannot bring themselves to believing in such a dichotomy. While the logic of if-then-this may be logical, as any philosophy professor will tell you, just because something appears logical does not mean it is correct.
So then, why evil? I wish I could say I had a simple silver-bullet answer, but I do not (I have discussed the subject here and here). If anyone claims a simple answer, run away fast. Despite the difficulty, there are many crucial observations that can come from asking this question.
C.S. Lewis once wrote, "My argument against God was that the universe seemed so cruel and unjust. But how had I got this idea of just and unjust? A man does not call a line crooked unless he has some idea of a straight line. What was I comparing this universe with when I called it unjust." In essence, when one denies the existence of God based on the existence of evil, it becomes a self-defeating argument. When we call something bad, we refer to a standard that is beyond ourselves. Where does that standard come from? If it is man-made, then it is subjective. Taken to its logical conclusion, crimes such as those committed in Darfur and Nazi Germany could be justified. In fact, the Holocaust was permitted by German law, although no decent person would say that such an atrocity was "good." If there is not a God, what then can we defer to as the arbiter when we say something is evil?
This does beg the question of how God communicates such standards to humans. Some would argue that humans can and do pervert the idea of divine revelation for personal gain -- if God "told" me that murder is evil, then what is to stop me from adding a few of my own pet-peeves to the list simply to control those I don't like? While humans have used the name of a religion for wrong, this is an ancillary criticism and never answers the question of where we draw our concept of right and wrong.
If God does exist, where does He come up with right and wrong? This is an interesting question offered by Plato, called the Euthyphro Dilemma. Simply put, Plato asks if an act is right because God says it's so, or does God say it's so because it's right. Some see Plato's question as a devastating objection to the concept that morality is grounded in God's commands, but it doesn't need to be. I saw one person make the following statement, "What if God had ordained murder and rape as the morally obligatory ways of treating others? If so and rights and values have moral authority merely because God ordains them, then murder and rape would be morally obligatory. You can't protest here and say that since God (who is perfectly morally good) would never ordain anything as immoral as murder and rape, murder and rape couldn't have been morally obligatory. If you say this, you'd be appealing to a moral standard independent of (not ordained by) God. So, either human rights and moral values have an authority independent of God's commands or they derive all of their authority from the fact that God has ordained them and we must accept that human rights and moral values are arbitrary: whatever God says goes - no matter how horrible."
Christian philosopher Thomas Aquinas argued that the dilemma is false (thanks Wikipedia for the following condensation of his argument): "Yes, God commands something because it is good, but the reason it is good is that good is an essential part of God's nature. So goodness is grounded in God's character and merely expressed in moral commands. Therefore whatever a good God commands will always be good." Thus, God's commands are not subject to an outside authority, nor are they arbitrary.
It is important to point out that someone does not have to be religious to do "good" things, like feed the poor (although, most outreach centers I can think of that offer such assistance are part of a larger church ministry). But as Norman Geisler says in his book Unshakable Foundations, "In order for moral evil to be present, a moral agent and a moral law must also exist." Who else can that moral agent be but God?