I think I understand why. It's fairly common in the U.S., where being Christian is generally considered to be a good thing.* Have you ever seen businesses advertise themselves as "Christian" or "Christian owned" and wondered, why the hell does it matter if a plumber is Christian or not?
It's because they're virtue signalling. To be Christian is good, it means you're more honest and have more integrity than non-Christians, and you want to signal to other Christians that you're one of the good people. I imagine that fic writer is doing it for similar reasons.
* A lot of people identify as Christians, but don't actually belong to or regularly attend any church except for perhaps (perhaps!) on holidays like Easter and Christmas. But they feel they're good people and they sort of believe in God even if they don't actually pray and haven't given religion much thought, so therefore... Christian.
Re: Weirdest tag I've seen in a while
It's because they're virtue signalling. To be Christian is good, it means you're more honest and have more integrity than non-Christians, and you want to signal to other Christians that you're one of the good people. I imagine that fic writer is doing it for similar reasons.
* A lot of people identify as Christians, but don't actually belong to or regularly attend any church except for perhaps (perhaps!) on holidays like Easter and Christmas. But they feel they're good people and they sort of believe in God even if they don't actually pray and haven't given religion much thought, so therefore... Christian.