Where my mind went reading that is the church being controlled by the devil, which... that's gotta be interesting. It'll also piss of a bunch of people. Plus, making a movie set in the Vatican? Expensive. Maybe it's a small town, maybe in the past, maybe something like Witch, it's America, The New World, where you're the only ones around for hundred of miles, and the Church is the heart of the town. And you're the new priest being brought in, and start to suspect that the leader of the Church has been possessed, or is just straight up evil.

Too late Scott! Northern Irish director Marty Stalker has done it. See http://m.imdb.com/title/tt2652948/ #HostagetotheDevil (2016)