Why not just "The Passion of Christ"?
Why is Mel Gibson's movie called The Passion of the Christ instead of The Passion of Christ?
Gibson likely chose the title for his movie because Christ is not a personal name but a title meaning "the Anointed One." So the title could be translated as The Passion of the (Anointed One). Had the movie been titled The Passion of Christ, it would have lost the sense that "the Christ" is a title, not a name, and could have been translated The Passion of (Anointed One), which is grammatically incorrect.