Aha my new friend, welcome to my favourite piece of Biblical real estate, the pre-denouement of the consummation of all prophecy and history.
Let us review the revelation of grace before the 7 plagues/ strikes, in the song of Moshe and the Lamb (a collaborative work):
Who shall not fear You, O Lord, and glorify Your name? For You alone are holy. For all nations shall come and worship before You, For Your judgments have been manifested.” (Rev 15:4)
What's this? ALL the nations? Surely those wretched heathens don't deserve it. Indeed, out come the plagues, down goes Babylon the harlot. 'Rejoice, the smoke of her torment goes up forever and ever.' (Rev 19:3). Yea she's history, gone daddy gone!
Then behold! King of kings and Lord of lords with the hosts of heaven in fine linen (shout out to F.L.) emerge to do battle with the forces of darkness. The beast and prophet captured and cast into the lake of theion/ sulphur and pur/ fire (Rev 19:20). And the dragon into the pit for 1,000 years, during which time the beast-free servants reign with Christ.
Happy days! Until Satan is released and quickly cons the nations again (o foolish nations!), gathering them for war from the 4 corners (note the flat earth reference in passing). They converge on Jerusalem but are consumed by heavenly fire (Rev 20:9), and the devil's sent to join his buddies in the lake.
So it's curtains for the nations? Not so fast, my friend, let's move on beyond the GWT judgment in Rev 20:15, where any unbelieving stragglers are scooped up and cast into the lake, along with death (the last enemy) and Hades.
Then into Rev 21, the new heavens and new earth, where God says 'See I am making everything new' (21:5). The renovations have begun. Then we get 21:7-8:
"He who overcomes will inherit these things, and I will be his God and he will be My son. But for the cowardly and unbelieving and abominable and murderers and immoral persons and sorcerers and idolaters and all liars, their part will be in the lake that burns with fire and brimstone, which is the second death."
And boy it's gonna hurt and be a surprise for many.
So by this stage all the nations have been (a) devoured by fire from heaven and/or (b) thrown into the lake by virtue of their inter alia 'unbelieving' ways. Agreed?
Next heaven descends to earth in the great marriage and the beauty of it is described. But next the theological bombshell:
The nations will walk by its light, and the kings of the earth will bring their glory into it. In the daytime (for there will be no night there) its gates will never be closed; and they will bring the glory and the honor of the nations into it; (21:24-26)
What just happened? The nations emerged from the lake in peace and worship, the fulfilment of OT prophecy going all the way back to Gen 18:18:
Abraham will surely become a great and powerful nation, and through him all the nations of the earth will be blessed.
The light to the nations has been switched on and illuminates the scene, the universal and manifold promises of all nations saved are fulfilled:
Youth With A Mission – “All Nations” Verse List
So we can understand Paul's great summation:
When all things are subjected to Him, then the Son Himself also will be subjected to the One who subjected all things to Him, so that God may be all in all. (1 Cor 15:28)
But hang on, not so fast there. Ppl still need to overcome in that there lake. Let go of the sin attachments, the love of darkness, let God's presence burn away the accrued dross of a lifetime of rebellion. They need encouragement...and they get it, in the form of a divine incentive package:
Blessed are those who wash their robes, so that they may have the right to the tree of life, and may enter by the gates into the city. Outside are the dogs and the sorcerers and the immoral persons and the murderers and the idolaters, and everyone who loves and practices lying. (22:14-15)
The Spirit and the bride say, "Come." And let the one who hears say, "Come." And let the one who is thirsty come; let the one who wishes take the water of life without cost. (22:17)
On either side of the river stood a tree of life, bearing twelve kinds of fruit and yielding a fresh crop for each month. And the leaves of the tree are for the healing of the nations. (22:2)
Just as prophesied in Daniel, Ezekiel and Isaiah, the river of life issues from the throne of God and flows out into the Dead Sea, bringing life to all things in it, and:
Fishermen will stand by the shore; from En-gedi to En-eglaim they will spread their nets to catch fish of many kinds, like the fish of the Great Sea. (Ez 47:10)
We are those fishermen, my friend. Get your rods and reels ready.
Because this is how it ends in Rev 22:21:
The grace of the Lord Jesus be with all. Amen.