If all the people who saw the movie, instead sat down and read the Bible for 2 hours, we would be living in a different world. But people don't do that. Even if they did, this movie would still offer great spiritual benefit.
Drama is the most powerful artistic medium. God created this art form. Using the theater to present Christ's Passion is the fulfillment of God's Will for the it.
Additionally, the movie makes it possible for devout Christians to understand Jesus' suffering more fully.
I truly believe this film was brought about by God to bring the Church to its knees (repententance) and spiritual revival. I think it will result in a certain number of people being saved, but I think its greater purpose is to bring Christians into a deeper relationship with Christ.