Psychosis and Culture

I was reading the Wikipedia page on delusions today (because that's how I roll), and came across this line, which made me chuckle:

[A delusion is characterized by:]

• certainty (held with absolute conviction)
• incorrigibility (not changeable by compelling counterargument or proof to the contrary)
• impossibility or falsity of content (implausible, bizarre or patently untrue)

A false belief based on incorrect inference about external reality that is firmly sustained despite what almost everybody else believes and despite what constitutes incontrovertible and obvious proof or evidence to the contrary.

Upon reading this, I said to myself "Hmmm, what does that sound like?" Not surprisingly, myself answered "Organized religion!" Wikipedia, alas, is more clever than I, though, and the very next sentence is:

The belief is not one ordinarily accepted by other members of the person's culture or subculture (e.g., it is not an article of religious faith).

Why is this, exactly? Why does a false belief's acceptance by more than one person make it not delusional, but a false belief held by just one person is indicative of madness? This is more of a rhetorical question, I suppose--I just thought it was amusing.