At what point does faith become insanity?

[Paleophyte]

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. 

Yes. Thank you. That was my point. Clearly these individuals are entirely sane with the context of their community. Equally clearly, the community as a whole has a few Daddy Who Art In Heaven Issues. As a group they are engaged in an activity which, when stripped of its ritualistic trappings, is typically lumped in with rape and murder. If you found somebody who had come up with this all on their own you would be right to be deeply worried. That this is a group behaviour does not improve the situation to my thinking.

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.

It is eternally amusing to me how one-sided your scepticism is. Let's give it the sniff test. Imagine this headline popped up, "Authorities in Florida have revealed that a small pagan cult believes that they can magically transform milk and cookies into the blood and flesh of Ashera, which they then consume to be closer to her..." How loud do you think the outraged shrieking would be?

Practicing behavior which is considered entirely normal by the community is not a sign of insanity, though from outside it might look that way.

What a lovely description of some of the worst atrocities that our species has ever perpetrated.