"appears to behave normally in regular matters and is capable of independent living, does not match other DSM symptoms, but believes insane things".
None of you think the major religions teach some insane things? Okay then.
“ Such ridicule typically will not occur after a religious conversion brings a previous non-conformist into the fold of the culture’s dominant religion.”
I was on the phone so couldn't type much, but my point is the concept of the collective cognitive imperative (much better explained in the book than in Wikipedia, to be fair) would answer your question by saying "it's not that nobody thinks that: it's that enough people accept those insane things to the point that 1) they don't think they're insane things, 2) you'd be an outsider (or even considered insane) if you point out the fact that they're insane things.
I'll try to give an example that would work in my cultural context (South America): someone making a rain dance during a drought would be considered by most to be something from harmlessly silly to crazy. Someone praying to the christian god for guidance or help would be seen as normal.
I mean, if they didn’t, then they probably would be religions, just “stuff everyone believes or has reasoned discussions about”.
Spoiler: they can’t all be right.
However, if I were to take a guess, it’s something like “people sometimes believe things without proof”. But obviously this is not true only of religious people, other people have their own creation stories – the sole difference is their’s don’t involve worship. And I agree they can’t all be right, but perhaps one of them is.
These are axioms, you cannot derive them, you can only derive from them. Saying “my axioms are rational, yours are ‘insane’!” is frankly childish, and speaks to a deep lack of understanding of the essence of reason.