What double meaning do you see in the exiles returning to Jerusalem and some serving as Levites? It is all in line with the other chapters up to this point.
20 And they shall bring all your brethren for an offering unto the Lord out of all nations upon horses, and in chariots, and in litters, and upon mules, and upon swift beasts, to my holy mountain Jerusalem, saith the Lord, as the children of Israel bring an offering in a clean vessel into the house of the Lord.
21 And I will also take of them for priests and for Levites, saith the Lord.
Jerusalem as being the heavenly;
REVELATION 3:12 He that overcomes will I make a pillar in the temple of my God, and he shall go no more out: and I will write upon him the name of my God, and the name of the city of my God, which is new Jerusalem, which comes down out of heaven from my God: and I will write upon him my new name.
REVELATION 21:2 And I John saw the holy city, new Jerusalem, coming down from God out of heaven, prepared as a bride adorned for her husband.
REVELATION 21:10 And he carried me away in the spirit to a great and high mountain, and showed me that great city, the holy Jerusalem, descending out of heaven from God.
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God's People as a Royal Priesthood
1 PETER 2:5
You also, as living stones, are built up into a spiritual house, a holy priesthood, to offer up spiritual sacrifices, acceptable to God by Jesus Christ.
1 PETER 2:9 But you are a chosen generation, a royal priesthood, a holy nation, a people for his own; that you should show forth the praises of him who has called you out of darkness into his marvelous light
REVELATION 1:6 And has made us a kingdom and priests unto God and his Father; to him be glory and dominion forever and ever. Amen.
REVELATION 5:10 And have made us unto our God a kingdom and priests: and we shall reign on the earth.
REVELATION 20:6 Blessed and holy is he that has part in the first resurrection: on such the second death has no power, but they shall be priests of God and of Christ, and shall reign with him a thousand years.
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2nd Coming
v15, For, behold, the LORD will come with fire, and with his chariots like a whirlwind, to render his anger with fury, and his rebuke with flames of fire.
MATTHEW 7:19 Every tree that brings not forth good fruit is hewn down, and cast into the fire.
MATTHEW 13:40 As therefore the tares are gathered and burned in the fire; so shall it be in the end of this world.
MATTHEW 13:42 And shall cast them into a furnace of fire: there shall be wailing and gnashing of teeth.
MATTHEW 13:50 And shall cast them into the furnace of fire: there shall be wailing and gnashing of teeth.
JOHN 15:6 If a man abides not in me, he is cast forth as a branch, and is withered; and men gather them, and cast them into the fire, and they are burned.
1 CORINTHIANS 3:13 Every man's work shall be made manifest: for the day shall declare it, because it shall be revealed by fire; and the fire shall test every man's work of what sort it is.
1 CORINTHIANS 3:15 If any man's work shall be burned, he shall suffer loss: but he himself shall be saved; yet so as by fire.
2 THESSOLONIANS 1:8 In flaming fire taking vengeance on them that know not God, and that obey not the gospel of our Lord Jesus Christ
HEBREWS 10:26-27 For if we sin willfully after we have received the knowledge of the truth, there remains no more sacrifice for sins, But a certain fearful expectation of judgment and fiery indignation, which shall devour the adversaries.
HEBREWS 12:29 For our God is a consuming fire.
REVELATION 14:10 The same shall drink of the wine of the wrath of God, which is poured out undiluted into the cup of his indignation; and he shall be tormented with fire and brimstone in the presence of the holy angels, and in the presence of the Lamb:
REVELATION20:10
And the devil that deceived them was cast into the lake of fire and brimstone, where the beast and the false prophet are, and shall be tormented day and night forever and ever.
REVELATION 21:8
But the fearful, and unbelieving, and the abominable, and murderers, and fornicators, and sorcerers, and idolaters, and all liars, shall have their part in the lake which burns with fire and brimstone: which is the second death.
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Just a few, I believe this to be the focus of Isaiah 66. As you know the Sabbath is an everlasting (forever) covenant so it seems to reason it will also be kept in the New Earth v22-23.