The reason atheists (and, I assume, communists) “bristle” at the comparison to religion is because that argument is merely step one of a larger argument, one we’ve seen hundreds of times before:
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Atheism is just another religion! Therefore, my (theistic) beliefs are just as valid as yours!
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Since my (theistic) beliefs are just as valid as your (atheistic) beliefs, one of us has to be wrong. I have a book which says I’m right, so your beliefs are wrong!
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Prophet!
Since this is clearly a bogus line of reasoning, it’s best to nip it in the bud when it starts.