I have not seen the movie -I'm looking forward to it.
But I wasn't expecting to see that much about Christianity since my understanding is that movie covers his early life up to the Folsom Prison concert - I have pretty much all of Johnny Cash's music and it didn't seem to have a very strong Christian theme at that point. There were hints of - like Grey Stone Chapel but his best religious expression came later - particularly the Man in Black album.
You are right about the fact that at the time of the Folsom Prison concert, Cash was not as deeply and openly religious as he was in later years when he was singing at Billy Graham crusades and what-not.
The other thing that should be remembered is that even as a "born-again Christian", Cash was edgy and never felt quite "safe" to those who wanted to pigeon-hole him in their image of what they thought a Christian should be. He didn't fit anyone's mold.
In particular, Cash never lost his sense of the importance of being real, and the importance of understanding and having on compassion on people right where they are in life. Cash certainly regretted the drugs and alcohol that he did and regretted the way he hurt those around him. But I never got the sense that he regretted having made any of the music that he did, since at root he was telling people's stories -- many of which are all too often sad, sordid, tragic, and R-rated.
There is a scene in "Walk the Line" that perfectly captures this sense of caring about people where they are, not where they should be. In it, Cash is discussing his proposed Folsom Prison concert with his record producers. I won't ruin it for you, but it was vintage Johnny Cash.
In the 90s, Cash was playing at acid rock festivals. I remember talking to a couple of teenagers who were going to travel to a big festival. There they were in their black leather and piercings. I asked them who was playing the concert, and one of the names they mentioned was Johnny Cash. Having last run across a Cash performance flipping past a Billy Graham crusade on TV, I did a double-take, assuming I had heard wrong. They told me, "yeah, he's gonna be there... he's the coolest."
When I asked them what they found cool about him, they replied. "He's the original man in black, man... the original man in black. Doesn't get any cooler than that."
And then there was the video everyone is discussing on this thread: "Hurt." Try to fit that one into someone's nice neat pigeon hole.
You will love the movie. When I first heard about the movie, I said, "Yeah, right." Then expectations started to build, and I was prepared to be disappointed. I wasn't.