The Exorcism of Emily Rose

Question: At the end of the movie, Emily is greeted by the Virgin Mary who gives Emily a choice. She can either ascend to heaven or remain on earth and become a martyr to prove that God and demons are real. Emily chooses to become a martyr and shortly after dies. How is her death supposed to prove that God and demons exist when Ethan came up with so many logical explanations for her demonic attacks?