This movie has all the elements for a great horror film, except for a plot with logical and consistent world building.
So the devil is after the fathers soul, and slowly kills him after years of ailing from some degenerative condition. So far, so good. You have an evil entity that slowly drains the life from someone to get their soul. But here's the issue; this same entity kills everyone else with ease.
The devil kills Louise, Micheal, the mother, the nurse, the priest, and rips apart dozens of sheep too. So this devil can kill, but it takes years to kill the father, ...for reasons. It also haunts the mother for years too, yet Louise and Micheal for only a week. Why? It's never explained or even hinted at.
This is an issue with many horror movies, especially ones with a supernatural element. Why does the evil antagonist toy with the characters, when the desired goal is their death, and this can be accomplished with ease. Some movies explain why: like feeding on fear to gain power, or fear makes victims taste better when consumed, or needing time to manipulate characters to release or summon them, or one of dozens of other explanations within the genre. This movie does none of that.
The audience needs to know the rules in a horror film, that's what creates the tension and anxiety. Rules let the audience know what's possible and what's at stake. If anything is seemingly possible, like an unstoppable devil that can kill anyone with impudence, then what are the stakes. It's an unfathomable force that kills half a dozen other characters at a whim, yet takes years to drain one man. There's no rules and no motivation given.
If you edited this movie to just have the "scary" scenes left, nothing would be lost. Realistically, you wouldn't even miss out on any real plot on context either. No motivations, no stakes, no rules, no plot: just scary things happen for unexplained reasons, the end.