http://www.imdb.com/title/tt0080100/ (IMDb says 1979 but the opening titles say 1978).
The soul of a young girl with telekinetic powers becomes the prize in a fight between forces of God and the Devil.
This is a little-known film with quite a few stars in it like Mel Ferrer, Glenn Ford, Lance Henriksen, John Huston, Sam Peckinpah, and Shelley Winters. Paige Conner is Katy, a scary little girl who seems to be a real sweetheart... but isn't. As far as I can tell, this is the only movie she was in. This has an all-American cast though it's an Italian-made film. Admittedly, it's not a great movie, with lots of overacting (did Shelley Winters ever not chew the scenery??), a horrible music score, and a disjointed story line, but it's one of those "Gee, I'd like to see it again because it might not be as bad as I thought" movies, besides which Paige is pretty, even when she's scary. From what I remember, this is definitely not on a par with Exorcist though it shamelessly steals chunks from all the demonic movies of the time (The Birds, Rosemary's Baby, The Omen, etc.), so don't get your hopes up.
The version I saw in the early 80's was in English on VHS but the several versions on eMule I've seen have Italian dubbing and no subtitles, and the DVD version is reputed to be longer with additional scenes. If anyone has the English version (or English subtitles for the DVD version), I would appreciate it.