I recently watched a debate between the Reverend Al Sharpton and the journalist Christopher Hitchens. They were (quite politely, actually) discussing the issue of the existence of god and religion.
It's important, in a way, to separate those two issues. It is possible for a non-religious person to believe that the universe was created by some intelligence. This begs the question of where this intelligence came from, but that is beside the point. It is one thing to argue against a religion, to point out the contradictions of a system of belief, or to showcase its benefits or cruelties.
But god? Can you argue against the existence of god? Not really. Nor can you argue for it. It's not really an arguing sort of thing. The non-believer asks, "Why would you believe that?" and the believer responds simply that he does. Either you have this vague feeling that such an entity exists, or you don't. A lot of this has to do with what you've been taught, especially in your childhood. Muslims tend to have Muslim children. Christians have Christian children. Hindus have Hindu children.
You may have been frightened in to your belief. You may have been bribed. You may have had a miraculous or mind-numbing experience that you simply couldn't chalk up to coincidence or explain logically. Maybe you get that tingling feeling like I get when the Millennium Falcon shows up at then end of Episode IV. Maybe you've just heard the same thing over and over again so many times that you just started believing it.
If you believe in a god, then there's no logical argument that can change your mind. I've seen many religious people turn non-religious. It's very rarely logic that does it, because religion is not a logical binding. Religion is an emotional, social binding created in a community (go ahead, look up the etymology). We can talk about the infeasibility of Noah's Ark all we want. That won't convince anyone of anything they didn't already believe. Religion is left behind when it no longer satisfies one's emotions. Some change religions, others leave it altogether. Our society, we can plainly see from statistics, is generally trending toward the latter.
This doesn't stop atheists, especially the recently dereligionized, from making long lists of logical problems with their former systems of belief. They seek reassurance and demonstrate shock at their former states just like anyone else going through a sudden change of emotional balance. How could I have stayed with that girl/guy so long? How could I have believed in astrology? Why was I smoking cigarettes all my life? Look how foolish I was!
So religions can be argued, point by point. But god? You can't really argue about it. You won't be able to prove it one way or another. You either feel it or you don't.