The Devils (1971)

Cardinal Richelieu and his power-hungry entourage seek to take control of seventeenth-century France, but need to destroy Father Grandier - the priest who runs the fortified town that prevents them from exerting total control. So they seek to destroy him by setting him up as a warlock in control of a devil-possessed nunnery, the mother superior of which is sexually obsessed by him. A mad witch-hunter is brought in to gather evidence against the priest, ready for the big trial.

http://www.imdb.com/title/tt0066993/

Director: Ken Russell
Stars: Vanessa Redgrave, Oliver Reed and Dudley Sutton