One of the main reasons I love this film is because it is a tour de force of Duvall's ability as an actor. He begins with a series of scenes which poison us against the main character of the film and then spends the rest of the film confusing our perception of him and then eventually helping us appreciate him. The performance is as complex as the character, infusing it with charisma and power and hypocrisy and, strangely, sincerity. Pacino and DeNiro had powerhouse performances in the 70s to display their full ability to work under directors who had free range to exploit the abilities of their character actors. Duvall, I feel, never had this opportunity in the golden age of director-guided films. This is his film, where he shows the full extent of what he can do as an actor.
The Apostle presents religion in all its best and worst, wrapped up in the single person, Sonny. In this one person we can see why religion deserves such respect and derision, why it is both powerful and silly, why it encourages both love and anger. All the contradictions, hypocrisies, miracles, lusts, glories and carnality of religion are on display here. Religion is humanity at its finest and lowest, and we see the full range of humanity. No, we are not all Sonny. But we are all somewhere in this picture, influenced by or repulsed by a person or community or organization that is just like him.
I love questions without an answer, but takes us down a long road while we consider the answer. The Apostle is a question like that for me.