Consider just the reasons for unbelief (I'm surprised, by the way, that the problem of suffering didn't make this list since it's probably the best reason for skepticism out there.).
1. Learning about evolution. There's no incompatibility between evolution and God. Many theists believe in both. In fact, as philosopher Alvin Plantinga points out, the conflict really lies between naturalism (atheism) and evolution. If evolution is true then our rational faculties have evolved to help us survive, not necessarily to lead us to truth.
Thus, those who believe both atheism and evolution to be true have no basis for trusting their reason to lead them to truth, especially metaphysical truth. But if that's so, then they have no basis for trusting their reason when it leads them to conclude that either atheism or evolution is true. If God exists, however, then we have grounds for thinking that God has caused our reason to evolve in such a way as to make it generally reliable.
2. Too many Christians doing unChristian things. Even if it were granted that many people fall short of what we might expect of them (who doesn't?) what does that have to do with whether or not God exists? God's existence doesn't depend on whether people who believe He exists live consistently with that belief.
3. Religion is the opiate of the people. Even if it were granted that religion misleads or stupefies many people that's also irrelevant to the question whether God exists. The truth of theism is one thing, the truth of a particular religion, or religion in general, is something quite different. It's ironic, parenthetically, that the opiate claim is taken from Karl Marx. If anything has stupefied the masses, as well as the intelligentsia, over the last one hundred years it has been atheistic Marxism.
4. Rational thought discredits religious belief. Even if it were granted that many religious beliefs cannot withstand rational scrutiny that has nothing to do with whether theism itself is rational. The claim that theism is discredited by rational thought is simply false as many, if not most, philosophers, both theist and atheist, have acknowledged.
5. Lack of evidence for a creator. This objection is as puzzling as it is common. There are numerous arguments that constitute evidence for a creator. At least two forms of the cosmological argument (the kalam argument and the argument from contingency of the universe), the argument from cosmic fine-tuning, and the moral argument are all strong arguments whose conclusions assert the existence of God.
6. Just don't believe it. I think this objection might better be stated, "I just don't want it to be true that God exists." If someone doesn't want theism to be true, of course, then nothing will persuade him or her that it is.
In fact, I suspect that #6, this deep desire for a naturalistic world in which people are not accountable for how they choose to live, is the fundamental reason for most unbelief today. The other reasons listed above certainly don't seem to constitute adequate explanations of it.
