Millenialism

Winston Zeddemore: Hey Ray, do you remember something in the bible about the last day when the dead would rise from the grave?
Ray Stantz: I remember Revelations 6:12. "And I looked, and he opened the sixth seal, and, behold, there was a great earthquake. And the sun became as black as sackcloth, and the moon became as blood."
Winston Zeddemore: "And the seas boiled, and the skies fell."
Ray Stantz: Judgment Day.
Winston Zeddemore: Judgment Day.
--Ghostbusters

Millenialism is the belief that there will be a period of time where Christ will return to Earth and reign before Judgment Day. In Revelations 20: 1-10, this period is proposed to be "a thousand years."

Various theories propose how this will materialize.

Amillenialism is the belief that the millenial reign of Christ is not physical; it is spiritual and symbolic. It is also currently active beginning with Pentecost, when the New Testament era's new church began. During the millenium, the spiritual kingdom of God will grow. However, political kingdoms of man will exert their power as well. This has led some amillenialists to posit that darks days of 'tribulation' await the Church until Judgment Day arrives.

Premillenialism is the belief that Christ will literally return to set up his kingdom--i.e., Christ will return prior to the millenium and personally administer his kingdom prior to Judgment Day. Historic, or post-tribulational, millenialists believe that the Church will experience a period of great tribulation prior to Christ's return. Dispensational, or pre-tribulational, premillenialists believe that Christ will return in secret to raise dead and living Christians into the sky (a.k.a. 'rapture') prior to the period of tribulation. After the period of tribulation, Christ will return with the raptured Christians to establish his kingdom and rule the millenium.

Postmillenialism is the belief that Christ's return will follow the millenial. It differs from amillenialism in that Christ's return to earth will be physical, and that the millenial period is associated with great evangelistic success of the Church, where billions of people will come to accept Christ as Savior. Until the world experiences a long period of Christian culture, Christ will not come again to end history and judge the world. Many postmillenialists have thus concluded that they need to 'save' and 'convert' others by any means necessary in order to hasten the millenial and Christ's second coming.