Making heaven and hell universally
available in the current world.
This would have taken away one of
the major sales points of medieval
Christianism and was hence declared heresy.
Robert Anton Wilson popularized
the slogan (as did The Shamen, UK
in "Destination Eschaton").

While jutta's definition is absolutely correct, it is common to misinterpret the phrase to mean "Imminentize the Eschaton", that is to bring the End of the World closer to hand. See Immanent and Imminent.

I would say that since one of the key elements of The Illuminatus! Trilogy evolves efforts to start World War III, that neither of the two Bobs (Robert Anton Wilson and Robert Shea) were clear on the difference when they wrote the books. They most likely scooped the phrase from The Principia Disordia.