This seems like good flavor for the setting. I don't know if I'd use it directly; it's possible to make a Macguffin out of it, but I don't know that I would. I'd be more likely to make the location, or the road to it, a setting for conflict involving the two dominant religions.
Of course, I could certainly see Trianarianists attempting to destroy such a tangible symbol of the dead prophet.Go to Comment
On reading this updated version I am upping my vote. The background made the character more real to me. His hidden skills in sorcery and necromancy, used as a tool in his battle for supremacy of his religion over the other, also speaks to the nature of the faithful in your world.
However, I do agree with the points made by valadaar. Saying "1 in 10" sounds like the simple mechanics of a die roll, which pulls me out of the story. I really like the idea of John in turmoil over being possessed by a spirit which both helps him and controls him.