What is the Millennium?
The Millennium is the literal one thousand year reign of Christ on the earth, the seventh dispensation of man, when Satan is bound and Christ and His saints rule in righteousness and peace before the final judgment and eternity begin.
The Millennium is described as the seventh dispensation of man, also called the Dispensation of the Kingdom. It is a literal one thousand years of rest for planet Earth, a time when all the promises of God to Israel and to the Church are fulfilled. Revelation 20:2-7 mentions the length of this period six times, showing that this is not a symbolic number but a real, literal reign of Christ on the earth. During this time Satan is bound and cast into the bottomless pit so that he can no longer deceive the nations, until the thousand years are fulfilled.
During the Millennium, the saints who did not take the mark of the beast are raised in the first resurrection and reign with Christ for a thousand years as kings and priests. Revelation 5:9-10 declares that the redeemed are made kings and priests unto God and shall reign on the earth, not merely in heaven. Christ Himself sits on the throne of His father David and reigns over the house of Jacob forever, as Luke 1:32-33 promises, and this earthly reign is where the sons of God rule and judge with righteousness alongside Him, as Psalm 72 describes: judging the poor with justice, breaking in pieces the oppressor, and ensuring peace for the people.
The condition of the earth at the beginning of the Millennium follows the wrath of the Day of the Lord and the War of Armageddon. Isaiah 11:4-10 pictures a renewed creation where the wolf dwells with the lamb and the earth is full of the knowledge of the Lord, and Isaiah 2:2-4 shows nations no longer learning war, beating their swords into plowshares. Zechariah 14:9 sums up the essence of this reign: the Lord shall be king over all the earth, and in that day there shall be one Lord and His name one. Global worship of Christ becomes mandatory over all nations during this period.
This reign is proven to be the Millennium, not eternity, because certain earthly features remain: the sun and moon still endure, people still die (though the righteous flourish and live long, as in Isaiah 65:20), and there are still seas and continents. In eternity, by contrast, there is no more sea, no more sun or moon needed, and no more death, as Revelation 21 and 22 describe. At the end of the thousand years, Satan is loosed again, deceives the nations of Gog and Magog to gather against Jerusalem, and is defeated by fire from heaven and cast into the lake of fire. This is followed by the White Throne Judgment, the resurrection of the wicked, and then the eternal new heaven and new earth.
The scriptures
And he laid hold on the dragon, that old serpent, which is the Devil, and Satan, and bound him a thousand years, And cast him into the bottomless pit, and shut him up, and set a seal upon him, that he should deceive the nations no more, till the thousand years should be fulfilled: and after that he must be loosed a little season. And I saw thrones, and they sat upon them, and judgment was given unto them: and I saw the souls of them that were beheaded for the witness of Jesus, and for the word of God, and which had not worshipped the beast, neither his image, neither had received his mark upon their foreheads, or in their hands; and they lived and reigned with Christ a thousand years. But the rest of the dead lived not again until the thousand years were finished. This is the first resurrection. Blessed and holy is he that hath part in the first resurrection: on such the second death hath no power, but they shall be priests of God and of Christ, and shall reign with him a thousand years. And when the thousand years are expired, Satan shall be loosed out of his prison,
And they sung a new song, saying, Thou art worthy to take the book, and to open the seals thereof: for thou wast slain, and hast redeemed us to God by thy blood out of every kindred, and tongue, and people, and nation; And hast made us unto our God kings and priests: and we shall reign on the earth.
He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David: And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end.
Give the king thy judgments, O God, and thy righteousness unto the king’s son. He shall judge thy people with righteousness, and thy poor with judgment. The mountains shall bring peace to the people, and the little hills, by righteousness. He shall judge the poor of the people, he shall save the children of the needy, and shall break in pieces the oppressor. They shall fear thee as long as the sun and moon endure, throughout all generations. He shall come down like rain upon the mown grass: as showers that water the earth. In his days shall the righteous flourish; and abundance of peace so long as the moon endureth. He shall have dominion also from sea to sea, and from the river unto the ends of the earth. They that dwell in the wilderness shall bow before him; and his enemies shall lick the dust. The kings of Tarshish and of the isles shall bring presents: the kings of Sheba and Seba shall offer gifts. Yea, all kings shall fall down before him: all nations shall serve him. For he shall deliver the needy when he crieth; the poor also, and him that hath no helper. He shall spare the poor and needy, and shall save the souls of the needy. He shall redeem their soul from deceit and violence: and precious shall their blood be in his sight. And he shall live, and to him shall be given of the gold of Sheba: prayer also shall be made for him continually; and daily shall he be praised. There shall be an handful of corn in the earth upon the top of the mountains; the fruit thereof shall shake like Lebanon: and they of the city shall flourish like grass of the earth. His name shall endure for ever: his name shall be continued as long as the sun: and men shall be blessed in him: all nations shall call him blessed. Blessed be the LORD God, the God of Israel, who only doeth wondrous things. And blessed be his glorious name for ever: and let the whole earth be filled with his glory; Amen, and Amen. The prayers of David the son of Jesse are ended.
For, behold, I create new heavens and a new earth: and the former shall not be remembered, nor come into mind. But be ye glad and rejoice for ever in that which I create: for, behold, I create Jerusalem a rejoicing, and her people a joy. And I will rejoice in Jerusalem, and joy in my people: and the voice of weeping shall be no more heard in her, nor the voice of crying. There shall be no more thence an infant of days, nor an old man that hath not filled his days: for the child shall die an hundred years old; but the sinner being an hundred years old shall be accursed.
But with righteousness shall he judge the poor, and reprove with equity for the meek of the earth: and he shall smite the earth with the rod of his mouth, and with the breath of his lips shall he slay the wicked. And righteousness shall be the girdle of his loins, and faithfulness the girdle of his reins. The wolf also shall dwell with the lamb, and the leopard shall lie down with the kid; and the calf and the young lion and the fatling together; and a little child shall lead them. And the cow and the bear shall feed; their young ones shall lie down together: and the lion shall eat straw like the ox. And the sucking child shall play on the hole of the asp, and the weaned child shall put his hand on the cockatrice’ den. They shall not hurt nor destroy in all my holy mountain: for the earth shall be full of the knowledge of the LORD, as the waters cover the sea. And in that day there shall be a root of Jesse, which shall stand for an ensign of the people; to it shall the Gentiles seek: and his rest shall be glorious.
And it shall come to pass in the last days, that the mountain of the LORD’s house shall be established in the top of the mountains, and shall be exalted above the hills; and all nations shall flow unto it. And many people shall go and say, Come ye, and let us go up to the mountain of the LORD, to the house of the God of Jacob; and he will teach us of his ways, and we will walk in his paths: for out of Zion shall go forth the law, and the word of the LORD from Jerusalem. And he shall judge among the nations, and shall rebuke many people: and they shall beat their swords into plowshares, and their spears into pruninghooks: nation shall not lift up sword against nation, neither shall they learn war any more.
And the LORD shall be king over all the earth: in that day shall there be one LORD, and his name one.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.