Religious fundamentalists are obnoxious because of how certain they are that their point of view is the only correct one and that every one else is completely wrong. I am always surprised by how similar, in tone, many atheist's posts are to those of fundamentalist religious extremists.
No matter what anyone chooses to believe, there is no way to prove anything about what God is or isn't or whether God exists at all. There is nothing wrong with having beliefs, as long as people are honest with themselves that their beliefs are choices and not verifiable, concrete fact. Agnostics, are the most logically correct, since they embrace the doubt and uncertainty inherent in the whole God debate.
Whenever anyone clings dogmatically to a belief, it is usually to try to convince themselves that they are not as mired in self doubt as they are, because the prospect of living with that uncertainty would make life meaningless for them, and that is to unbearable for their sense of self-importance to cope with.