At what point does faith become insanity?

[Belacqua]

social norms

People have been taking mass for quite a long time now. In their community, it's completely normal. It is, therefore, a social norm. 

For people raised within that community, it is not insane or irrational to hold to that social norm. 

You judge it to be opposed to some essential, basic part of human nature (an anthropological taboo), and therefore not sane. I am skeptical of that. Practicing behavior which is considered entirely normal by the community is not a sign of insanity, though from outside it might look that way.