Those types of badfic almost always betray their setting entirely - no matter what the premise of the story is, "Everyone and their grandmother converts to (a very, very specific subset of a denomination of) Christianity" is always jarring. I mean, you could throw that plot at Narnia and it would throw everyone off, for pity's sake.

(Though not quite as badly as people who attempt to retrofit the entire canon to fit their specific idea of the world/Christianity. That's worse. is, to this day, one of the only 'fics I've ever seriously considered missioning.)

So honestly, even giving a canon doesn't narrow it down - your best bet is to try and find the site you found it on first, and search key words in that fandom until something comes up.