That's not true. I read the book and saw the movie. Both were excellent. The majority of the book was about his surviving the unsurvivable. At the very end of the book, it tells of Louie's transformation at the hands of Billy Graham and God. So it is part of the book but a small part. The point of the book is his rise as a runner, to surviving on raft in the Pacific and the sadistic torture by the Japanese. "Unbroken" is about him enduring and surviving that which most men could not. There is no big story to tell if it was only about him finding the Lord at the end. The meat of the book is the raft experience and the Japanese tortures.
To dismiss the movie because it does no dwell on the Billy Graham aspect of the book is ludicrous. It is a movie, not a 10 hour epic. All movies shortchange the books they are about because you can't put the entire 400 pages of the book into a two hour movie. Jolie wanted to focus on the inhuman experiences Zamperini had to endure and survive as a result of the plane crash and the Japanese POW camps. Saying the movie should have focused on the end of the book when Louie found Christ, is disingenuous, at best.
Maybe you could go see the sequel where they focus on that part of the book.