Let’s start with a meaning of echatology:
- The branch of theology that is concerned with the end of the world or of humankind.
- A belief or a doctrine concerning the ultimate or final things, such as death, the destiny of humanity, the Second Coming, or the Last Judgment.
The following copies-and-pastes various (OK, darn near all) the sections on this topic at Wikipedia [I would note that I have edited out the links and various indications of problems with the text below]:
In political theory and theology, to immanentize the eschaton means trying to bring about the eschaton (the final, heaven-like stage of history) in the immanent (existing) world. In all these contexts it means “trying to make that which belongs to the afterlife happen here and now (on Earth)”. Theologically the belief is akin to postmillennialism as reflected in the Social Gospel of the 1880–1930 era, as well as Protestant reform movements during the Second Great Awakening in the 1830s and 1840s such as abolitionism.
Modern usage of the phrase started with Eric Voegelin in The New Science of Politics in 1952. Conservative spokesman William F. Buckley popularized Voegelin’s phrase as “Don’t immanentize the eschaton!” Buckley’s version became a political slogan of Young Americans for Freedom during the 1960s and 1970s.
Voegelin identified a number of similarities between ancient Gnosticism and the beliefs held by a number of modern political theories, particularly Communism and Nazism. He identified the root of the Gnostic impulse as belief in a lack of concord within society as a result of an inherent disorder, or even evil, of the world. He described this as having two effects:
- The belief that the disorder of the world can be transcended by extraordinary insight, learning, or knowledge, called a Gnostic Speculation by Voegelin (the Gnostics themselves referred to this as gnosis).
- The desire to implement a policy to actualize the speculation, or as Voegelin said, to Immanentize the Eschaton, to create a sort of heaven on earth within history. See Scientism.
One of the more oft-quoted passages from Voegelin’s work on Gnosticism is that “The problem of an eidos in history, hence, arises only when a Christian transcendental fulfillment becomes immanentized. Such an immanentist hypostasis of the eschaton, however, is a theoretical fallacy.”