The girl Emily Rose sacrificed herself and she chose to suffer a lot to show that demons are real because she said in her letter which she wrote in the movie that Holy Mother of God "Virgin Mary" met her and She asked her to choose whether she would like to go with her and live peacefully or stay like this with demons inside her and suffer greatly to show the world that demons are real. But Emily chose to stay like that. The most emotional line of the letter in which Emily says: "People say that God is dead but how can they think that if I show them the devil". Emily suffered greatly and died to show the people that God is real and she loved God so much. A very beautiful and emotional story I would say its one of the best exorcism story.