You have to differentiate between the arguments for the existence of god and arguments over the validity of organized religion.

As far as god goes, the argument is moot.
God, as most people use the term, refers to an all-powerful being, omnipotent, omnipresent, omniscient, etc. The human mind is a finite thing, incapable of conceiving of the infinite, so how can one begin to prove or disprove the existence of an infinite being?
It cannot be said for certain either way, so arguing about it is pointless.

Now arguing about the validity of organized religion--christ, if people can't figure out when they are lying to themselves, how do you expect them to know when they are hearing the truth?

Man is fallible. Man wrote the bible. Man is capable of lies and deceit, and of being deceived.
Why trust in the works of man, when he tells you the work is "of god?" Everything is of god, and being of god makes the bible no more a source of truth than Beowulf, or Le Morte De Arthur, or any other myths. Take Scientology. It has fiercely dedicated followers that believe what is an obvious pack of poorly written lies put together by a failed Sci-Fi author. What kind of horseshit is that?!