The “Millennium” is the 1,000 year reign of Jesus and His church on earth. Revelation 20:6, “…they shall be priests of God and of Christ, and shall reign with Him a thousand years.” There are definitely many differences in the symbolic heavens before and after Christ’s reign.
The air or the heavens represent the spiritual order over the earth. Satan is the usurping spiritual ruler of this present evil world. Ephesians 2:2, “…prince of the power of the air, the spirit who now works in the sons of disobedience.” It is Satan and his fallen angels who established false church systems, false theologies, the antichrist, and systems of lies. Jesus returns to destroy these powers. Isaiah 34:4-5, “And all the host of heaven shall be dissolved, and the heavens shall be rolled together as a scroll: and all their host shall fall down, as the leaf falls off from the vine, and as a falling fig from the fig tree. For my sword shall be bathed in heaven… upon the people of my curse, to judgment.” These scriptures picture Jesus’ judgment on the sinful spiritual organizations.
During the millennium, Christ’s righteous rule will be established. King David said, “Let the heavens be glad, and let the earth rejoice, and let them say among the nations, “The LORD reigns!” 1 Chronicles 16:31. “…we are looking forward to a new heaven and a new earth, where righteousness dwells.” 2 Peter 3:13(NIV)
After Christ’s millennial reign, Satan will be released from the bottomless pit to test restored mankind. Revelation 20:7, “And when the thousand years are ended, Satan will be released from his prison.” Those whose hearts do not love God and righteousness will reveal their sinfulness. Then Satan, the fallen angels, and sinful men will all be destroyed in second death. Revelation 20:9,10, “fire (final destruction) came down from God out of heaven, and devoured them (men still aligned with sin). And the devil that deceived them was cast into the lake of fire and brimstone (symbolic of complete destruction), where the beast and the false prophet are.”
Finally, Jesus will deliver restored, righteous mankind back to God, His Father. 1 Corinthians 15:24-28 (CSB), “…then the end, when he hands over the kingdom to God the Father, when he abolishes all rule and all authority and power. For he must reign until he puts all his enemies under his feet. The last enemy to be abolished is death…When everything is subject to Christ, then the Son himself will also be subject to the one who subjected everything to him, so that God may be all in all.”
God’s rule will continue forever in the “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.” Isaiah 65:17, 18.