An endorsement by The National Evangelical Association will bring the Assemblies of God and other evangelical Christian groups on board to see the film. Focus on the Family has a radio show that is widely listened to by born-again Christians. For instance, if you're the type of person who would wear a T-shirt saying "Jesus Saves", then you probably listen to Christian radio and TV and will be open to hearing a story about Gibson's movie.
In other words, this was a VERY smart move by Mr. Gibson, because the above-named organizations will give positive feedback on the movie that will energize their home base to see the film.
I saw him on an interview on EWTN and he looked like he'd make a fantastic Jesus: beautiful features, long dark eyelashes, soft, expressive eyes, gentle but masculine voice. Thought he was great in "Count of Monte Cristo" too.
He also has the correct initials: JC -- hehehe.