I am pretty sure this was "inspired" by the Jonestown incident. In 1988, it says 13 years agothe mass death occur in 1975. In 1978, Jim Jones and his followers committed mass suicide/murder in the multiple hundreds.
I hate IMDB's Signature policy

Or the Heavens Gate cult. But that was after this movie came out.

No, and it wasn't inspired BY any either
Seriously, I think you may be confusing the events at the beginning of the film with the true events at Waco, Texas circa 1992. There is also a generic resemblance between the onscreen cult members and other true cultists, like the Jim Jones Cult or The Manson Family.
The script writer may have watched some episodes of Dateline or 20/20, but no true event is credited to have inspired this film.

And that's the kind of day it is here in the mind of FRIGHTFAN!