Interesting in this context is completely divorced from morally good/bad. Could be any group from any area at any time in history. I’ll start with a few, followers of the cult of pythagoras, contemporary black Hebrew Israelites, antiracist skinheads and the Amish (neo-luddites in general). Don’t be racist or a prick to other people discussing.
Birds aren’t real is another joke that people starting taking seriously like flat Earth.
I remember when both were joke subreddits. Then it turns out that if you have enough people pretending ridiculous shit some of them will take it seriously.
The KKK has a similar origin story.
Ultimately, this is how Scientology started.
A pub bet, to a running joke, to a mass delusion we allow to continue because not doing so would subject the old world power structures (“religion”) that are still operating today to the same scrutiny.