How plausible is the setting?

I think this is a good film and I'm not criticizing it for lack of "realism," but I do wonder how many holes could be poked into the notion of a Scottish island in fairly modern times abandoning Christianity in favor of a pagan system (especially to the extent of practicing human sacrifice.)

For one thing, it seems unlikely that the clergy who left Summerisle would keep silent that its inhabitants were becoming "heathens" en masse, and that nobody associated with the Church of Scotland or the Catholic hierarchy would care that for many decades there's been no functioning church on an island in an otherwise fervently Christian region.

Another is that Lord Summerisle's grandfather presumably had research assistants or at least people to help plant trees and memorize instructions on ensuring the apples grew in a hostile climate. If these were all outsiders who returned to the mainland at a later date, then that's yet more persons who kept totally silent their entire lives about an extraordinary mass conversion. But if they were islanders (and even if they weren't) it seems really unlikely that everyone is so incredibly ignorant of horticulture and has the mentality of a Bronze Age tribesman who can only think to improve a harvest by appeasing the gods.

I'm sure there's other examples people could bring up, but I want to reiterate I don't think it detracts from the film or its themes. After all, lots of great cinema wouldn't exist if total realism were required.