It's not bad
Exorcism movies are hard to do. You walk a fine line between horror going full evil dead, theology for entertainment, contorting actors, and using prayer as the primary battle against the demon possessing the person. It's especially hard with modern audiences being used to physical altercations with monsters and demons.
The bad: it's a Hollywood exorcism movie. You're getting cliché CGI, lighting, music, and pacing. It's a tight runtime so the audience doesn't really experience a realistic depiction of the time and process. It's always a race against time and the world is at stake. This movie falls into this. The movie needs to be shocking and scary rather than thoughtful, eerie, and tense. Because of this, the family isn't interesting and you don't really worry about the boy.
The good: Russell Crow has a loveable character in his depiction of Amourth. His candor is different from the usual moody dark priests we usually see on movies like this. He carries this movie and it's a great performance.
Having read a lot on exorcism and books by Exorcist, its sad that the depiction is rarely handled well by studios. They're pushing the horror elements which become typical and laughable. When it came to the original Exorcist, the audience grew to care about Reagan before fearing the demon possessing her. The Exorcism of Emily Rose also did this well. Unfortunately, the Pope's Exorcist is missing this. We barely know the family and that works against it.
I'd give it a 6 out of 10. Crow was great.