1. The ostensibly evangelical Protestant population of America is pretty large, and I think most every one of them is going to see the film at least once.
2. IF Mel holds to his original idea NOT to put in subtitles, there is a large number of this same population which is going to be familiar enough with the gospel accounts of the passion that they are going to view the film as a kind of puzzle: Can I figure out from simply watching the screen when Jesus is saying this or that? Or Peter is saying so and so? and so forth. It is this part of the experience which I think is going to generate tons of repeat viewings by evangelicals.
3. The Catholic population of America is going to see the film, every one of them at least once, since Catholics (much less Christians generally) seldom get much favorable media attention. Mel is going to become a folk hero for conservative to moderate Catholics (though I expect the liberal Catholics are going to sprout a forest of hemorrhoids over this film).
Again, the more Biblically literate among them are going to watch it multiple times, comparing the film as they watch it with the gospel account which they already know (or read just before seeing the film).
What is going to be REALLY INTERESTING is to watch what happens at the Oscars with this film. How deliscious!! If they praise the film, they damn themselves. If they ignore the film, they damn themselves!! Mel is cooking up a conundrum for them comparable to what Jesus often cooked up for the Pharisees.
My prediction: one of the top 10 grossers of all time.