St. Thomas Aquinas (1274: I, q. 1, a. 3) gave two objections to belief in God:

The problem of evil and unnecessary hypothesis are still the main objections to belief in God. My response emphasizes moral ideas. I'll defend God's moral goodness, against the problem of evil; and I'll argue that God's moral purposes are part of our best explanation of why the world exists as it does, against the unnecessary hypothesis.

Why Evil?

With all the evil and suffering in the world, is it reasonable to believe that a good and perfect God created the world?

I live in Chicago. Here many evils happen every year – deaths, diseases, poverty, robberies, tornadoes, and so forth – bringing physical pain and mental suffering. Similar evils take place all over the world. Now imagine that there's a being X, who either caused these evils or could easily have prevented them. X had the power to bring only joy to people, if he had so chosen, but instead brought about (or at least permitted) massive evils. Here X, of course, is God; Bertrand Russell says that X would be a moral monster if he existed. Should we agree?

Here's a traditional formulation of the problem of evil:

The first two premises are based on taking an “all good being” as one who wants to do good and prevent evil as far as he's able – and an “all powerful being” as one who's able to do anything that's logically consistent.