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 evidence render your entire belief system fundamentally flawed?