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No one hates a particular religious group quite like the people whose own religion is 99.9% identical to theirs (e.g. Catholic vs. Protestant)
I think that the ridiculousness (ridiculosity?) of other's religions reflects the believer's own religion's absurdity, made especially worse, in my opinion, if it's only recognized on a subconscious level.
Like in the movie Fight Club when the narrator says he can't cry with another faker around.