Don't let any genre indications fool you, this is not a horror film. I was prepared (country of origin, release date, plus the poster gave it away) when I started watching, so no real disappointment there, but unless you only care about plot points it's hard to defend calling this a horror film.
A priest is sent to a small town to investigate a case of possession after a local priest took advantage of the situation. Once there he meets Mother Joan, who is possessed by the devil. As he tries to deal with the situation as best as possible, the priest starts to struggle with his own faith and convictions.
Ultrastark camera work, empty frames (the decor is literally walls), and an absent soundtrack turn this into a very minimalist experience. The story nor the characters appealed to me, the styling was grim and unpleasant and the runtime excessive. Dreyer meets Bresson, with a tinge of Bergman and Tarkovsky. Take the worst aspect of these four, mix them together and you get Mother Joan of the Angels.