Disclosure: I am an atheist.
I do believe that theists are welcome to go on believing if that’s what gets them through the day. I think that religion has some nice crispy edges that talk about how to basically live well and do right by your fellow man. Granted, it threatens those who don’t with eternal damnation as incentive to stay on the straight and narrow, but taken at its core as a set of guidelines on how to be a decent fellow, it’s got its heart in the right place, so if people want to take it more or less at face value because it gives them direction and hope, then so be it.
Having said that, I can’t place in a supreme being in a place that is equivocal to not believing in a supreme being. They are not both beliefs like any other. Atheism is about what your senses and the scientific method tell you is real. Can it be detected? Is its effect measurable in any useful way? What, exactly, does it do? How does it interact with everything else? And so on. The answers to these questions, provided they have factual answers, form the basis of what we know, not what we believe. Anything that doesn’t have clear answers forms a theory based either on circumstantial evidence or educated guesses – but even those have grounding in reality that is extrapolated from existing known facts. Dark matter is a theory because we can’t really see it or prove its existence, but its existence can be inferred from the grouping and placement of stars and galaxies, and gravitational anomalies that couldn’t be there without something pushing all this other stuff out of its way. We don’t know what it is, but we know it’s something because what we do know about the universe just doesn’t make sense without it.
But you can’t do that with religion. Nothing can be proven by the scientific method because there’s nothing to test, nor is there any evidence we can point to in order to even infer any theoretical models about it. All we have are words in books – old books with translated words. Anecdotal evidence at best, which even itself is subject to interpretation and debates on which is the correct translation based on context.
So what religion boils down to for an atheist is, “You’ll just have to trust me on this one.” It becomes a logical fallacy, argumentum ad populum; billions of people can’t be wrong. For anyone who relies on logic and critical thinking to define their world, this is a non-answer, and furthermore runs contrary to the that very model. Something that cannot be proven, or even theorized by inference, cannot be accepted as any form of truth, and anyone who believes otherwise is being illogical.
Humans on the whole have a tendency to be illogical though, so it is something I accept, even as I disagree with it.