One of the key problems for atheists and believers in some divinity (God, source, force or purpose) in the universe is the question of evil. The atheists use the existence of evil as proof that there is no god or that if there is one, he/she/it is a failure because, in the argument of Harold Kushner’s book, When Bad Things Happen to Good People (1981), God is benevolent but not able to prevent evil. God is simply not all-powerful. Neither atheists nor many Christians want to believe that. Atheists simply deny the existence of a god while Christians find it hard to accept a less than omnipotent one.
Believers have frequently attributed evil to an external source (e.g. Satan, the Devil). If you take this route, you have a dualistic world view: God vs. the Devil. This was the view of the Persian religious leader Zoroaster, who posited a God of Good and one of Evil. This view eventually worked its way into Jewish and then Christian thought.
Another Christian view is that evil exists as a test, helping us grow spiritually by giving us an opportunity to prove ourselves worthy by choosing the Good. As humans, we have free will and thus can choose evil over good, sometimes seeing evil as apparent good. Probably the most common “free will” explanation of evil is made more deterministic by the fifth century Christian theologian and “church father” Augustine of Hippo. He locked in the important but unfortunate doctrine of Original Sin to explain, at least to the Western Christian world, why evil exists.
It is our fault, not God’s. So there! We all inherited the sin of Adam and Eve.