Millennial Reign Answer:
In the theology of Revelation, the millennium is tied very closely with the parousia. This is shown by the fate of the devil, who finally shares the same fate as the beast and the false prophet but only after a delay of a thousand years. The consequence of the parousia is the destruction of all evil, but the destruction of evil at its deepest level is portrayed NOT as an immediate consequence, but as one delayed a thousand years.
Before asking WHY this is, we should notice that another effect of the millennium is to separate one aspect of the last judgment (Re 20:4) by a thousand years from the last judgment itself (Re 20:11-13). Comparing Re 20 with one of its major sources, the vision of the divine judgment in Da 7:9, we see that the thrones of Re 20:4 come from Da 7:9, and the opening of the books in Re 20:11 from Da 7:10.
Dan 7 concerns the destruction of the beast that has persecuted the people of God, and the transference of his kingdom to the Son of Man and his people. THIS is what is depicted in Re 19:11-21 (the destruction of the beast) and Re 20:4-6 (the transference of the kingdom to the saints). The negative aspect of the final judgment (Re 19:11-21) requires as its positive counterpart that judgment be given in favor of the martyrs, who must be vindicated and rewarded.
In the contest between the beast and the witnesses of Jesus, the beast SEEMED to triumph, and the martyrs to be DEFEATED. When the heavenly perspective prevails on earth (let your kingdom come!) so that the truth of all things becomes evident, not only must the beast be seen to be defeated, but the martyrs must be seen to triumph. And just as the kings of the earth who shared the beasts usurped rule are deprived of their kingdom, so the martyrs now reign with Christ.
If you read carefully, you will see that what is said about the martyrs in Re 20:4-6 is strictly limited to what contrasts with the fate of the beast. The martyrs evidence, with Christ's, has condemned him. But the divine court vindicates the martyrs. He has been thrown into the lake of fire (19:20) which is the second death (20:14), but they come to life and the second death has no power over them (20:4-6). The kingdom is taken from him, and it is given to them. Now the destroyers of the earth have been destroyed (Re 11:18), and the earth is given to Christ's people to rule with him (Re 20:4 cf. 5:10; Dan 7:18,27).
Life and rule these two issues on which the contest between the martyrs and the beast focuses are the SOLE themes of Re 20:4-6; they are merely stated without elaboration.
The theological point of the millennium is solely to demonstrate the triumph of the martyrs that those whom the beast put to death are those who will truly live (eschatologically) and that those who contested his right to rule and suffered for it are those who will in the end rule as universally as he -- and for much longer: a thousand years!
To show that the martyr's triumph in Christ's kingdom is NOT one which evil can again reverse (that it is God's last word for good against evil) the devil is given a last chance to deceive the nations again (Re 20:7-8). But this is no re-run of the rule of the beast. The citadel of the saints proves impregnable (Re 20:9).
John gives the image of the millennium a very specific function. He used it to depict an essential aspect of his concept of the martyrs' victory over the beast. Once the image is taken literally -- such as predicting an actual period in the future history of the world -- it becomes impossible to limit the function of the millennium as John does. Then we have to ask all the questions which interpreters of Revelation ask about the 'millennium,' but for which John gives NO ANSWER, since they are of no interest to the function for which he uses the 'millennium' in his symbolic universe.
Not recognizing that the 'millennium' is a literary device to contrast the ends of the beast and of the martyrs, people are left to ask whom the saints rule, whether they rule from heaven or earth, how eschatological life of resurrection is compatible with an un-renewed earth, who are the nations Satan deceives at the end of the millennium, and on, and on and on. The millennium becomes incomprehensible once we take the image literally.
But there is no more need to take it literally than to suppose that she sequences of judgments (seal openings, trumpets and bowls) are literal predictions. John no doubt expected there to be judgments; but his descriptions of them are imaginative schemes designed to depict the theological MEANING of God's judgments. John expected the martyrs to be vindicated. But the millennium depicts the meaning, rather than predicting the manner of their vindication.
Blessings!
Covenant Heart