The setting of the main story project I'm working on, which is also the setting for most of the role-playing I've done, has a Christian-focused faction in it, but I wouldn't really call my setting conventional, so some of the advice I could suggest to you wouldn't be entirely appropriate. Concepts like demons are also recurrent to an extent in my work, though, again, they're not conventional. On the example you used involving destroying demons with faith, that particular element also exists in the project, but it's also a bit unorthodox and the way that faith works is a bit different too. In short, I have plenty of advice, but I don't know how relevant any of it would be since I don't know how typical you want the setting of this role-play to be.

Perhaps the most appropriate advice I can give you involves the portrayal of the faith itself. I'm not a Christian (at least not in the conventional sense), but part of my own philosophy required me to gain a lot of insight into all the religions (and all of their branches), and the first mistake I can tell you that most role-players, or even just writers in general, make is this; getting the portrayal of the religion accurate in the first place. If you're not a practicing Christian yourself, I think the most important thing is to do your research. Learn about what the followers are supposed to believe in, the values they're meant to hold onto and live according to in everyday life, and anything else that will make the portrayal believable. There's far too many works of fiction out there that try to portray a religion (I'll be blunt here, nine out of ten times they just do it to bash the religion) and the way it's presented feels incredibly fake.

So that's my biggest bit of advice to you; make sure you have the fundamentals right. Unless the character in question is supposed to be a false believer (because, realistically, not all people who call themselves Christians are true Christians), I generally portray the characters who follow a faith in my story very accurately to a true follower, whether those characters are affiliated with the faction I meant earlier or just independent believers. If you'll find it useful, I can potentially send you a PM with more specifics on that matter, if bringing up the specifics of various faiths is too troublesome here. I don't really know exactly what sort of information you're looking for based on your opening post alone, and I'd prefer not be too public for the time being about revealing my own work.