There's an issue with this
idea though. How do you know your god is the right one? A normal
atheist retort to this is to ask something like, “what's the
difference between me believing in Yahweh and believing in Zeus?”
They'll usually respond by either defending Yahweh (or whatever God
of choice) as the true god which is very hard to do without any
proof, or they'll respond with a universalist / pantheist argument
that all gods could be (or are) derivative of the same god or force.
I think this argument makes more sense than trying to prove your god is right using only your sacred documents, however there are some serious flaws. If they're fundamentalist and give this response they're definitely violating their dogma, however if they interpret their holy book more liberally it's more reasonable. This is slippery slope though. Once you claim the book can be interpreted differently over time you have to see how far this can actually go. To go from the idea that Jesus is the son of the one true god to Jesus is the son of one of the many derivatives of god is to move quite far from the original intent – not to mention arguments that the trinity actually makes Christianity polytheist.
Usually, as an atheist, you stop here and argue that since there's no proof for god and the books are highly flawed that god probably doesn't exist. However, for some reason I took the fallacious argument of “no evidence for absence” to the next level – insisting that if all gods lead to the same place, maybe there's a rationalist god, call Him “Flying Spaghetti Monster”, that actually doesn't want to be known, worshiped, etc. The only way to honor Him is to try to act as rationally as possible. There is no evidence for absence, right? I know that I always get a good feeling when acting rationally – I'm sure this must be His saucy Spirit guiding my way. If you look around, you can see the beauty of His fruits of rational living: curing the sick, uplifting the poor, etc. The holy sacraments of noodles and tomatoes are clearly good for you – look how healthy Italians are!
Of course, this is ridiculous, but in fact it's really the Christian argument without the history – and I should add without any contradictory history either. I really don't see the difference, but I'd love to hear why I'm wrong!