"I saw the souls of those who had been beheaded because of their testimony for Jesus and because of the word of God. They had not worshiped the beast or his image and had not received his mark on their foreheads or their hands. They came to life and reigned with Christ a thousand years (The rest of the dead did not come to life until the thousand years were ended.)" (Revelation 20:4-5).
The thousand year period referred to in Revelation 20 is known as the millennium. Christians are in agreement that the millennium is a scriptural concept, but they disagree as to the nature of the millennium.
Premillennialism teaches that Christ will return first, followed by a literal one-thousand year reign of Christ on earth. This will be the period when many Old Testament prophecies concerning the nation of Israel will find fulfillment. The Church will have been raptured prior to, or at the moment of, Christ's return. When the thousand years are ended Christ will put down a final rebellion, which will be followed by the creation of a new heaven and a new earth.
Amillennialism teaches that there will not be a literal one-thousand year reign of Christ. The kingdom of Christ and the kingdom of Satan exist contemporaneously until the time that Christ returns; which will coincide with a general resurrection, judgment, and establishment of the eternal kingdom.
Postmillennialism teaches that through the preaching of the gospel and conviction of the Holy Spirit, people will repent and the world will become Christianized. After this, Christ will return, the resurrection and judgment will take place, and the eternal kingdom will be established.
While these views differ greatly, each is considered to be within the realm of orthodox Christianity. Each affirms that Jesus Christ will one day return to judge the living and the dead. It is not an issue to break fellowship over.