What is the reason for the 1,000-year reign of Christ?

TL;DR:

The thousand-year reign of Jesus, or the millennium, is a period between the tribulation and the world's destruction, where Jesus will rule over the earth. This thousand-year reign fulfills God's promises to Israel, Jesus, the Gentiles, and all of creation.

what does the bible say?

The thousand-year reign of Jesus, also known as the millennium or the millennial kingdom, is a period of time between the tribulation and the destruction of the world, when Jesus will reign over all the earth. The 1,000-year reign of Christ serves to fulfill God's promises to Israel, Jesus, the Gentiles, and all of creation. During this period, God's covenants, including the Davidic, Palestinian, and New Covenants, will be fulfilled, with Jesus reigning from Jerusalem. This reign will also address the restoration of creation, bringing peace to animals, productivity to fields, and freedom from disease. Despite the peace and worship of God during this time, many will still choose not to follow Jesus, proving humanity's need for God’s guidance. Ultimately, the 1,000-year reign is a time to see God’s promises fulfilled, and while we wait, believers have the opportunity to share the gospel and invite others to repentance.

from the old testament

Throughout Israel's history, God has made several unilateral (one-sided) promises, or covenants, promising to bless Israel in specific ways. Some of these covenants have yet to be fulfilled. The Palestinian Covenant (Genesis 15:18–20; Numbers 34:1–12) outlines the exact borders Israel will possess for eternity; the nation of Israel has yet to encompass these specific boundaries. The Davidic Covenant (2 Samuel 7) is the promise that God made to David that David's heir would sit on the throne of Israel forever. Jesus is that heir, but He has yet to take His physical, political birthright. The New Covenant found in Jeremiah 31:31–34 outlines the promise that Israel, as a nation, will return to God and worship their Messiah (see Romans 9–11). These covenants will be fulfilled during the 1,000-year reign.
In Psalm 110, David records the promises that God ("The LORD") made to Jesus ("my Lord"). God invites Jesus to sit at His right hand until God makes Jesus's enemies a footstool. Jesus will rule over, judge, and shatter His enemies while His people worship Him freely. This combination of judging enemies and being worshiped by the faithful cannot happen simultaneously in the eternal heaven because in heaven, there will be no sin or rebellion to judge. Similarly, In Daniel 7:11–14, Jesus is given dominion over all the people, nations, and languages before His enemies are permanently destroyed.
The promise in Daniel 7:11–14 refers to Gentiles as well. It says that Jesus will be given dominion over all the nations. The whole world will know God (Isaiah 11:9).
God also made promises that apply to creation. Animals will live in peace again (Isaiah 11:6–9; 35:9; 65:25). Fields will be productive and freed from the curse of weeds (Isaiah 32:13–15), and the mortals who survived the tribulation and populate the 1,000-year reign will be freed from disease (Isaiah 33:24; 35:5–6; 61:1–2; Ezekiel 34:16).

from the new testament

In the promises given to the Gentiles, with Jesus ruling over them, God also promised that His faithful followers will be given positions of authority (Luke 19:12–27; Revelation 5:10; 1 Corinthians 6:2). This will not be fulfilled until the 1,000-year reign.
Regarding God’s promises to all creation, we also await the day when the curse of sin and death will be lifted (Romans 8:18–23).

implications for today

During the millennial kingdom, it will be clear that humanity needs God. Even though there will be peace throughout the world, even though the worship of God will be the norm for all nations, even though Jesus Himself will reign from Jerusalem, and even though Satan will be bound and unable to influence people, there will still be those who do not choose Jesus as their Lord and Savior. For all those who wonder if Adam and Eve would have rebelled against God if the serpent hadn't tempted them, the end of the 1,000-year reign reveals the answer. When Satan is released in Revelation 20:7, the nations are completely open to his deception and willing to follow him into battle against Jesus one last time. This is not just a few individuals, but their "number is like the sand of the sea" (Revelation 20:8). The influence of Jesus over civilization and the removal of Satan are not enough to encourage people to follow God. Only the Holy Spirit dwelling in our hearts ensures we will choose good. For now, as we await God’s fulfillment of His promises to Israel, Jesus, the Gentiles, and creation, we can take to heart. Second Peter 3:8–9 says, "But do not overlook this one fact, beloved, that with the Lord one day is as a thousand years, and a thousand years as one day. The Lord is not slow to fulfill his promise as some count slowness, but is patient toward you, not wishing that any should perish, but that all should reach repentance." The 1,000-year reign offers the opportunity for God to fulfill His promises; the wait offers the opportunity and responsibility for us to share His gospel with others and invite them into repentance.

understand

The 1,000-year reign fulfills God's promises to Israel, Jesus, the Gentiles, and creation, restoring the earth.
The 1,000-year reign will give people the opportunity to repent and come to Jesus, with Satan bound and Jesus reigning.
The future 1,000-year reign calls believers now to share the gospel and call others to repentance.

reflect

How does understanding the 1,000-year reign of Christ influence your perspective on God's promises and faithfulness?
How can you share the gospel and invite others to repentance, knowing that the reign of Christ is coming?
How does the reality that even in a perfect world, some will reject Jesus teach you about God and humanity?

engage

What are the specific promises God made to Israel, and how do they connect with the fulfillment of His covenants during the 1,000-year reign?
How does the fact that Jesus will rule the world and judge His enemies challenge the way we understand God's justice and mercy?
Why do you think some people, even in a time of peace and Jesus's reign, will still choose to reject Him? What does this tell us about the nature of humanity's need for God?