This was basically Duvall's pet project, he's also the writer and director.
It has a handful of scenes that work extraordinarily well where the degree of difficulty is off the charts. The film is a delicate balancing act that could have gone of the rails any number of times and somehow works the whole way.
Fundamentally you have to believe a) that this completely crazy pentecostal preacher is not a fraud and b) that he could believe himself to be an apostle of god while on the run from the law for a murder that you personally watch him commit and that other people wouldn't think he was insane despite strong evidence that he was a criminal background. That's a hard movie to play seriously, and it's a hard movie to make without cheating or falling into cliche religious scenes. The characters are complex while the plot is not. Frankly, the film should have had a nomination for best original screenplay.