One issue I must confront is this: if atheism is correct, why are there so many theists? I believe that, aside from history, there are two main emotions guiding people to theism: fear and love. Since the fear is primarily fear of death, I sometimes refer to these as "love and death" in homage to Woody Allen.
Fear
Face it, if you are religious, one of the appeals of religion is that you don't want to die. I don't blame you. I don't either.
But everything we know about minds points to the fact that a mind, whatever that is philosophically, just can't exist without a functioning brain. We lose consciousness even while we sleep, and our brain is mostly working then. General anesthesia disables only a few crucial parts of our brains, yet our minds do not experience anything while we are under. There's loads of scientific research correlating brain activity with mental events. So the idea that our minds might somehow continue to function long after our brains have gone to dust, however appealing, must be wrong.
But how appealing is it really? Eternity is a really long time. If you are reading this, chances are you have lived less than 100 years. Can you imagine, after death, your mind continuing to "live" for a thousand years? I can, and it might be pretty nice. How about ten thousand years, i.e. a hundred human lifetimes? A hundred thousand? A million? The thing about eternity is, however long it's been so far, you've barely begun. At some point you'll have played every possible chess game and solved every possible Sudoku puzzle. The Sun will eventually explode, and billions of years later the Universe may reach its heat death. But almost all of your years would still be ahead of you. There's no way out. Sounds like Hell to me.
Sure, human life is painfully short, but we might as well make the most of it!
Love
We humans (and probably the rest of the mammals too) love to love and love to be loved.
Why? I imagine it started with parental love. Certainly by the time there were mammals, baby mammals had to feel something akin to love to keep them with their parents. Love of one's parent probably conveys an evolutionary advantage; critters who love their parents are more likely to obey them, which tends to keep them out of trouble.
A human child's love for its parent is huge: the child's sense of self is almost completely tied up with the parents' approval. The parent appears benevolent, omnipotent and omniscient. The child's feeling toward his parents is a lot like reverence. This reverence must be a good thing evolutionarily.
For a while. Eventually the child must make his own way. Gradually, the child's sense of self becomes less tied up with his parents' approval, and the child recognizes that the parents are not always benevolent, nor are they omnipotent or omniscient. But the field of psychiatry tells us that this transition is not easy for the child (or the parent either, but that's not relevant here).
Evolution has a way of reusing things; it's often more practical to use something for a new purpose than to invent something new for that purpose. I believe (and I am hypothesizing here) that evolution took advantage of the giant hole left in the heart of the adolescent by the loss of reverence for his parents by reusing that reverence for (the human invention of) God.
Why might it be evolutionarily advantageous to believe in God, if God doesn't exist? There's a lot of speculation about this, but surely it promotes clannishness (my god is better than your god) which in turn might promote survival.