Horror masterpiece? Really?
I have to assume that if you think this film was scary or horrifying in any way, that you have to believe that organized religion or people who have strong religious believes are threatening or disturbing to begin with. Being a strong Christian person myself, I have no problem with those who try to convert or "save" another human being from eternal damnation. If you choose to believe that there is no God, no heaven, or no hell that is your personal opinion; it is not threatening to me just as the main character in this "horror" film was not scary to me.
On the contrary she was about as threatening as your typical monk or religious order of the medieval age. She never once killed anyone or harmed anyone until the very end of the movie, and even that was when she felt threatened by an unholy demon. There were the hints that she had killed another patient but that's up for debate. I was more freaked out by the transformation of the terminally-ill woman than anything else in the movie. And you could see that coming from a mile away. Which brings me to another criticism: the whole thing was totally predictable.
The moments in the film, like the levitation scene and the speaking to God in Welsh (oh boy what a horror that is) are at worst, the same old same old, and at best, interesting to watch as far as character analysis goes. But as far as horror is concerned they are pretty tame as far as horror films are concerned. They're just blips on a screen.
To say that this movie is a horror masterpiece is really a stretch. Maybe the viewers who feel threatened by the prospect of being brought closer to God will find it scary, but I can think of way more horror films that made this movie, by comparison, about as harrowing as Mary Poppins.