What sets this movie apart from other southern themed movies of the era is that it doesn't really feel dated. Perhaps owing to the mysterious Brother John character, who's aloofness puts the entire scenerio under a microscope and lends itself very well to viewing the movie 30 years later! Brother John is 99 percent drama one percent science fiction. the general tone and Quincy Jones soundtrack is "gentle 70's" to paraphrase a sarah silverman episode. poitier's character is like a black james Bond home after solving some hugely important case. His love interest played by Beverly todd is the films other main asset as she is beautiful and charismatic.
here is the only trailer
I could find. Sorry couldn't link it to here (not youtube).
note: there is apparently a biblical/ Poitier as Christ subtext that has helped to make it a, pardon the expression, cult classic to many.