Spoilers.
TL:DR: The Movie lacked nuance and subtlety. Where they could've made an interesting movie that compares faith and demons to the struggles of mental health and trauma, they instead opted for a wannabe-horror movie that was actually a poorly made action thriller.
First off, the movie wasn't terrible. The sets, though limited, were decent, the costumes were simple but effective, and the acting was perfectly passable. Sadly, that's all the movie was: passable. The action starts from the get-go with little to no suspense or subtlety, and the plot and dialogue seem to lean heavily pro-catholic.
This catholic perspective is used to push back against people who "believe in science" making them seem foolish, but by the end, Anne defeats the demon by accepting her trauma and taking control of her life and body once more, semi-siding with said science-believers.
The plot was fast-paced with no suspense or weight. Constant time jumps awkwardly explain the entire plot, spelling it out for the viewer as if its simple plot was that hard to understand, and adding previously non-existent events to explain the main character's connection to every possessed person in the movie. The lack of characters, especially ones with names, leaves the cast pretty sparse, as well as easy to puzzle together their "hidden" connections.
The plot twist had no impact, and barely any setup if you can call it that. There was not a clear antagonist, or even a "force" to fight against, making it hard to root for Anne as the hero of a villainess story. Had they fleshed out the idea of the devil, and played him up as the internalized negative emotions people face when struggling with mental illness, it could have been at least interesting. They never even explain why the devil targets her specifically, other than she's devout and gave a baby up for adoption.
Instead, an attractive blonde woman defies the traditional church, beats the unseen devil, changing from a dress to a suit as well, and shows the church that a woman can be an exorcist because she uses her feelings.