And to ensure replies: This is one of the reasons why I don't believe in God.
All those people all thought they had the one true religion bit down. They believed in Zeus or Odin and their creation stories. Or thought that the world was flat and carried around on a giant turtle flying through space.
What makes Christians, Jews, and Muslims so sure they got it right this time when so many others were "wrong" and eventually faded from existence once another more attractive religion came along?
Not to mention that since it happened before, it'll surely happen again and that Christianity, Judaism, and Islam will be replaced with something else.
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It's not even the metaphysics and ontology which is the problem with religion. It's the epistemology and ethics of it. Everybody is an agnostic, and the only people who try and truly deny that are dogmatic fundamentalists. Having a belief that there is a extra-natural superintendent is fine. Having the belief, or pretending to have the knowledge, of precisely who this superintendent is, is where it crosses the line.