It's arguable that the original believers of religion were insane (e.g. shamans with schizotypical personality disorder, temporal lobe epilepsy, etc...), yet with each subsequent believer in your culture, you are less and less insane to believe in it.

But this would be true only if the subsequent believers were not taking into account previous believers as evidence - if they had all come to the same view independently. Otherwise we have an information cascade.