I've seen a couple Christian-themed FPS games but they sucked. No doubt simply lack of talent in the maker; I'm sure there's not something inherently lame about the subject matter. Plenty of D&D tropes come straight from various religious texts.
Although I will say that when I read the blurbs for the game, watch the trailer, read reviews, watch LPs ... if I see religious stuff I generally just turn it off and do not buy. I thought Diablo was a fine game despite its religious overtones. Probably would have enjoyed it more without them. I shouldn't have to sit there writhing in intellectual agony, whether it's a game, a movie, a book, etc. But that's not just religious content:
For example, I just went to the theater to see Battle Royale Hunger Games. All but one of the movie trailers I gave a big damn thumbs-down to because they all looked so stupid. And this is the best light they can possibly put the movie in. The Hobbit Part 2 was the only one I would see, and I think the movies would have been so much better if they would just stop trying to invent connections between The Hobbit and LotR, stop parading in the old stars, stop it with all the elves already, and drop the albino completely. Hopefully someone out there makes a fan edit of the Hobbit films without the extra shit. It would probably fit into a three-hour viewing too.
TL;DR: I have no patience for bullshit.