What will the nations do in the Millennium?
During the Millennium the nations will worship Christ as King, keep the feast of tabernacles, live in peace, and be judged and ruled with righteousness by Christ and His saints.
When Christ sets up His Kingdom after the War of Armageddon, the nations that remain on the earth will be required to worship Him. Scripture says every nation left after the battle against Jerusalem will go up year by year to worship the King, the Lord of hosts, and to keep the feast of tabernacles. Any family or nation that refuses to come up to worship will suffer consequences: no rain will fall on them, and if Egypt in particular refuses, they will be struck with plague. So worship of the Lord becomes a yearly, mandatory practice for the nations that survive into this era.
The nations will also live under a rule of global peace and righteousness. Christ will judge among the nations and settle their disputes so completely that they will beat their swords into plowshares and their spears into pruninghooks; nation will not lift up sword against nation, nor will they learn war anymore. Kings and peoples from distant lands, even those from Tarshish, the isles, and Sheba, will bring gifts and presents, and all kings will fall down before Him and all nations will serve Him. Those in the wilderness will bow before Him, and even His enemies will lick the dust, showing His total dominion from sea to sea and from the river to the ends of the earth.
Before the nations settle into this pattern of worship and peace, they first face a time of judgment. When Christ returns, the surviving nations are gathered before Him and separated like a shepherd divides sheep from goats. Those on His right hand, who cared for the hungry, thirsty, stranger, naked, sick, and imprisoned, are called blessed and inherit the kingdom prepared for them, while those on His left, who neglected these things, are sent away into everlasting fire prepared for the devil and his angels. The saints who reign with Christ take part in this judging of the nations, along with judging those born during the Millennium and those raised in the second resurrection.
Once set right, the nations that remain will experience the restoration of the earth itself. The land that lay desolate will be rebuilt and inhabited, cities will be established, vineyards planted, and gardens made, and the heathen who are left will know that the Lord builds the ruined places and plants what was desolate. The overall condition among the nations will be one of righteousness flourishing, abundance of peace as long as the moon endures, and the earth being full of the knowledge of the Lord as the waters cover the sea.
The scriptures
And it shall come to pass, that every one that is left of all the nations which came against Jerusalem shall even go up from year to year to worship the King, the LORD of hosts, and to keep the feast of tabernacles. And it shall be, that whoso will not come up of all the families of the earth unto Jerusalem to worship the King, the LORD of hosts, even upon them shall be no rain. And if the family of Egypt go not up, and come not, that have no rain; there shall be the plague, wherewith the LORD will smite the heathen that come not up to keep the feast of tabernacles. This shall be the punishment of Egypt, and the punishment of all nations that come not up to keep the feast of tabernacles.
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.
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.
When the Son of man shall come in his glory, and all the holy angels with him, then shall he sit upon the throne of his glory: And before him shall be gathered all nations: and he shall separate them one from another, as a shepherd divideth his sheep from the goats: And he shall set the sheep on his right hand, but the goats on the left. Then shall the King say unto them on his right hand, Come, ye blessed of my Father, inherit the kingdom prepared for you from the foundation of the world: For I was an hungred, and ye gave me meat: I was thirsty, and ye gave me drink: I was a stranger, and ye took me in: Naked, and ye clothed me: I was sick, and ye visited me: I was in prison, and ye came unto me. Then shall the righteous answer him, saying, Lord, when saw we thee an hungred, and fed thee? or thirsty, and gave thee drink? When saw we thee a stranger, and took thee in? or naked, and clothed thee? Or when saw we thee sick, or in prison, and came unto thee? And the King shall answer and say unto them, Verily I say unto you, Inasmuch as ye have done it unto one of the least of these my brethren, ye have done it unto me. Then shall he say also unto them on the left hand, Depart from me, ye cursed, into everlasting fire, prepared for the devil and his angels: For I was an hungred, and ye gave me no meat: I was thirsty, and ye gave me no drink: I was a stranger, and ye took me not in: naked, and ye clothed me not: sick, and in prison, and ye visited me not. Then shall they also answer him, saying, Lord, when saw we thee an hungred, or athirst, or a stranger, or naked, or sick, or in prison, and did not minister unto thee? Then shall he answer them, saying, Verily I say unto you, Inasmuch as ye did it not to one of the least of these, ye did it not to me. And these shall go away into everlasting punishment: but the righteous into life eternal.
Thus saith the Lord GOD; In the day that I shall have cleansed you from all your iniquities I will also cause you to dwell in the cities, and the wastes shall be builded. And the desolate land shall be tilled, whereas it lay desolate in the sight of all that passed by. And they shall say, This land that was desolate is become like the garden of Eden; and the waste and desolate and ruined cities are become fenced, and are inhabited. Then the heathen that are left round about you shall know that I the LORD build the ruined places, and plant that that was desolate: I the LORD have spoken it, and I will do it.
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.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.