The book of Revelation describes the millennium earthly kingdom:
1Then I saw an angel coming down from heaven, having the key to the bottomless pit and a great chain in his hand. 2He laid hold of the dragon, that serpent of old, who is the Devil and Satan, and bound him for a thousand years;3and he cast him into the bottomless pit, and shut him up, and set a seal on him, so that he should deceive the nations no more till the thousand years were finished. But after these things he must be released for a little while. 4 And I saw thrones, and they sat on them, and judgment was committed to them. Then I saw the souls of those who had been beheaded for their witness to Jesus and for the word of God, who had not worshiped the beast or his image, and had not received his mark on their foreheads or on their hands. And they lived and reigned with Christ for a thousand years. 5But the rest of the dead did not live again until the thousand years were finished. This is the first resurrection. 6Blessed and holy is he who has part in the first resurrection. Over such the second death has no power, but they shall be priests of God and of Christ, and shall reign with Him a thousand years. 7 Now when the thousand years have expired, Satan will be released from his prison 8and will go out to deceive the nations which are in the four corners of the earth, Gog and Magog, to gather them together to battle, whose number is as the sand of the sea.(Rev. 20:1-8, NKJV)For a thousand years (taken figuratively to mean “a long time” by some) Satan will be bound and the saints shall rule the earth with Christ.
The is the only passage in the bible that speaks of a millennium.
Postmillennialism, like premillennialism, teaches there will be a millennial kingdom. The difference is in the timing of Christ’s second coming. In postmillennialism, Christ’s second coming is after the millennium. In premillennialism, His second coming is before.
Part of the explanation for this difference in chronology comes from the previous chapter in Revelation, summarized by the rider of the white (pale) horse:
And I saw heaven opened, and behold a white horse; and he that sat upon him was called Faithful and True, and in righteousness he doth judge and make war. (Rev. 19:11, NASB)Both postmills and premills agree that the events described in Rev. 19 occur prior to the events of Rev. 20 (the millennium) but they differ greatly in their interpretation. Premills interpret Rev. 19 as the second coming, and since it occurs before the millennium of chapter 20, well that’s what makes them premills. Postmills interpret the events of Rev. 19 as signifying the ultimate victory of the church on earth, not the second coming. It symbolizes victory of the gospel and the ultimate success of the great commission to evangelize the world.
To the postmill, Christ will rule during the millennium but it will be from heaven, and the millennial kingdom is largely spiritual and redemptive in nature.. For the premill, Christ will physically be on earth during the millennium, and the kingdom physical and political.
Postmillennialism, as discussed previously, is by its nature optimistic about human history, and this is a logical necessity given their interpretation of Revelation 19. To the postmill, the New Testament church is the transformed Israel, the "Israel of God" that Paul wrote of to the Galatians:
Peace and mercy to all who follow this rule, even to the Israel of God.Consequently, Postmills attach no eschatological significance to the nation of Israel. The gospel will spread throughout the world, and Christianity will strengthen, not weaken. Many nations will be Christianized as the great commission succeeds in winning converts, including Jews, to Christ. Through the supernatural work of the Holy Spirit, the postmill foresees gospel prosperity.
(Gal. 6:16, NIV)
So what ushers in the millennium? Here is a big difference with premillennialists. For the postmills, there is no catastrophic event signaling the onset of the millennium, there may even be no discernable discontinuity at all. (Indeed, some believe we may already be in the millennium period.) Christianity will expand; at some point the millennium begins, perhaps with defeat of the antichrist, and Satan (having been largely defeated by the gospel) is bound. The difference between the culmination of the church age and the millennial kingdom may be as much one of extent as of substance. At the end, Satan is freed and a great apostasy ensues which is terminated by Christ’s second coming, a literal resurrection, and His judgment--followed by the final form of the Kingdom.