"Millennium" means "one thousand years." The Bible specifically mentions a thousand-year reign of Christ in one passage (Rev. 20:1-6). In a vision, John there observes Christian martyrs come to life and then rule with Christ for 1,000 years. Jesus earlier warned of "ten days" of persecution which would include martyrdom for some (Rev. 2:10). Here Jesus rewards "ten days" of suffering with "1,000 years" beside him on the throne -- both time periods (as I understand them) being symbolic. Others note the same contrast but take the time periods literally.

Since the second century, many Christians have anticipated a Golden Age of 1,000 years on the present earth beginning with the return of Christ. During that time, they believe, Christ will fulfill many Old Testament prophecies by ruling over the whole earth in perfect justice and peace. The early church fathers Justin Martyr and Irenaeus held this "premillennial" view, although Justin Martyr notes that many pious believers of his day did not. After the Roman Empire became officially "Christian," Augustine concluded that Christ's thousand-year reign was already taking place. That (spiritualized millennial) view has not been shared by believers who suffered instead of reigning. Some other believers look for the Millennium before Jesus comes back, as the result of the gospel's progress and the church's work. That "post-millennial" belief spurred Alexander Campbell to devise a plan to "restore" ancient Christianity.

The precise meaning of the 1,000-year reign in Revelation 20 is not at the core of the gospel, and it is not something about which those who love and follow Jesus Christ should break fellowship or engage in combat. The important point is that we be found in faithful company with Jesus. The Lamb of God who once was crucified for our sins will surely come again as King of kings and Lord of lords, and God's kingdom finally will fill the new heavens and earth and endure forever.