"Christian Vampire" it is
I am thinking that a guy is demonized (in flashbacks?) and lives a long long time (more than 100 years?) as a vampire, or at least thinking (deceived) that he is one. Don't know which way to go on that count.
I am thinking that my hero probably needs to be Catholic, as the rituals of the church are part of the vein I am tapping into . . . exorcism? baptism? communion? These all have symbolic (and visual, artistic) meaning that the Protestant equivalents lack. But I may give him a Protestant sidekick -- somehow.
I am thinking that I will set it in the future, so there is the opportunity to hypothesize about the future of the world and church. Maybe that is how the Catholic and Protestant work together.
Maybe on a misison field?