@Masaki HarunaYou may not know it, but quite a few things are inherently inspired by Christianity. I think most of it's pretty subtle, but the more overt ones are pretty much anything angels and demons related.
I think, depending on what you're going for, it can either just be any other RP or it'll cause a shitstorm.
7 Deadly Sins/7 Heavenly Virtues is cliche and used a lot.
Demon Hunting
Anything supernatural related will either focus on cryptids and mythical monsters (think bigfoot to wendigos) or it'll go the whole spectrum of Vampires to Demons to Angels. But, also, if you're going for something Christianity related, it's not only going to hit Christianity. If you're using things from scripture, especially angels or holy figures, you're likely going to touch base on Islam, Judaism, and Christianity, as there are plenty of points where they both mix. Plenty where they don't, either, such as Djinn or the general concept of angels and demons that a Jewish friend explained to me in Judaism.
Regardless, I think you're safe to use most everything concerning Christianity as long as you don't actually touch figures like Jesus or even the Apostles. I'm a practicing Christian, myself, and the only thing that makes me uncomfortable concerning writing and Christianity is if Jesus is mentioned or used, at all, as anything. It's not offensive, it just makes me squirm a bit; doesn't sit well with me, personally, if I'm involved.
But, I'm also
fascinated by everything and anything occult, so I'm extremely interested in angel and demon lore, in particular.
If you're talking about using Christianity as the entire basis of an RP, I don't... think that would be an issue? It wouldn't be for me, in particular. People are free to have their own opinions, but I still think respect goes a long way. So, even if your RP is satirical or lowkey making fun of Christianity, you can still be respectful about it. It's hard, but it's possible. Just, in general, respect for anything culture or religion-wise in your writing goes a long, long, long way. So, if it's anything that uses Christianity intensively, then I'd go to great lengths to research everything, even the things that might not be necessarily canonical.
If it's just a bunch of demon killing and angels and all that, then, really, it's all up to you. So many things draw from a religious standpoint, so whatever you're doing that involves these things can have a basis and inspiration drawn from Christianity without
being Christianity. A good example of this is WH40K's space marines and all those heretic memes floating around.
As a hard list, though.
Do's:
-Fill in the lore gaps (angels and demons have very malleable lore; they're changed on a whim; research scripture on various faiths about them, if you like)
-Lessen your mentions about God; make faith either more community oriented or more personal. It's about how faith affects a character, or a group of characters, or an organization, rather than the faith itself.
-Research. Research. Research. Read passages from the Bible, Quran, Torah, etc. Even if you don't believe in them, they're great inspiration for fictional works.
-The Good vs. Bad is a tried and true concept. Don't be afraid to use it, but also don't be afraid to subvert it. There are grey areas and making everything grey works extremely well too, if you know how manipulate perspectives. For help on this, try reading up on the crusades; it's easy to think you're fighting for the good guys, but they're the actually the baddies. It's hard to string it along and make it seem like they're still the good guys, if not... a bit misguided.
-Have an idea of what your plot is before diving head long into filling everything with Christianity. Having a simple idea is best. A goal, a conflict, a setting. And then putting in the other information; you'll have a better idea of what in Christianity fits where and if it even works at all.
-Being vague leaves things to the imagination, but know when to be vague and when to give necessary info.
-Depending on the situation and the RP, I think it's best to either work for/with the Christian faith or against it. Having too many variables can muck things up unnecessarily and that includes allowing people to play as villains. But, I'm biased because I hate having things split down the middle. For me, I only ever enjoy having one solid group. Whatever works for you and the RP (but, if your RP is plot intensive and linear, I'd heavily suggest you either disallow villains entirely or allow people you trust to play them. This is purely because having someone you don't know be in charge of a specific plot point and how it goes is very, very tricky)
Don't's:
-Try to stay away from using specific religious figures. Saints are okay, as they are numerous and plentiful, and can essentially be created canon or not. This also dives into Catholicism. But, try to avoid people like Jesus or Peter, or Matthew unless you have a solid idea of who they are and what they represent and can do so tastefully. (research is your best friend)
-Sects. Don't label your specific idea of Christianity. Keep it vague to avoid putting yourself into a restrictive box, concerning ideas and canon.
-I keep saying it, but it's what works for this instance: keep the idea of God vague. Like I said before, it's about the characters and their faith, not about the faith itself.
-Don't just wing it. Have an idea of what you're doing. More importantly, have an idea of what your backstory is for whatever world you're in. If it's just modern day, then have a backstory for whatever you're putting into the world 'n shit.
-Don't brush people off in your OOC when they're concerned about something and it affects them mentally. In this day and age, it's pretty easy to just wave off things when people get offended, especially in writing because "It's fiction, so what's it matter?" It's easy to be a dick, but it's also easy to tank your reputation with some great writers. Be willing to change things if people are finding it unsavory. You don't have to, but it's common courtesy and will earn you a lot of respect from a lot of people. There's a difference between caving in or pandering and being a decent person. In other words, don't be a dick about this shit, especially if it's not something you're actually a part of or practice, in real life.
That's all I have, though. Just remember, I'm one person and this is just advice, so take it all with a grain of salt. It all is really dependent on what you're going for. Some things really don't need a do/don't advice thread, like 7 Deadly Sins Roleplays or ones in the supernatural genres. But I do understand where you're coming from: you want something that's guaranteed to work.