Theological term used to describe the act of bringing about the Kingdom of God or Dominion of Heaven in the immediate future; usually translated into American as "trying to cause the end of the world."
Eg., Joachim of Fiore was guilty of immanentizing the Eschaton, and the Protestants have been at it ever since.
In the Illuminatus! trilogy, a history of religious and political upheaval in the last century by Dr. Robert A. Wilson,
a the Illuminatti attempts to immanentize the Eschaton by resurrecting a horde of nazi shocktroopers preserved at the bottom of a lake, and killing the attendees of a music festival in the woods in Bavaria. Some broad with a lot of free time on her hands trips hard enough to become Eris, and throws her golden apple (which is becoming dated, if you ask me.) and the nazis all kill each other, or somesuch. And some dolphins make a pontoon bridge across a bottomless lake. It was far out, man.