This question is partly inspired by Question 2170.
There are obviously a great many specific arguments against theistic belief, but in general, most (as far as I can tell) boil down to the claim that there is not enough, or perhaps any at all, rational evidence for the existence of God, and since a rational person should only admit to those things for which he has an adequate amount of rational evidence, a rational person should not believe in God.
Specifically, the claim seems to be that we should only "believe" in something if we first have rationally convincing evidence for it to be true. But, even if I acknowledge that there is little to no rational evidence for God's existence, does it necessarily follow that to believe in a deity is irrational?
Put another way, is it possible to have a logically consistent theistic belief system against which the only argument is that there is not enough evidence to prove it to be true? Does the simple act of believing in something for which you don't have...
I think that the best argument against the existence of God is the standard one: if he as great as the leading religions make out, he would not have created a world like this one. Of course it is not a demonstrative argument. But I haven't come across a plausible response to it. But accepting your premise, I'd say: in order rationally to believe in something you need some evidence or argument for it. After all, it's easy to come up with endlessly many ludicrous hypotheses that are consistent with the evidence available to us: e.g. the world is densely populated by very small, very clever, very fast little green men who take care to remain undetected. Why believe in God and not believe in them? You need a reason.