The purpose of the millennium was to give the Jews time to accept Christ before God’s judgment on them in AD 70 (Matthew 21:33-45; 22:1-14; 23:29-39; Luke 21:5-33).
Here is a snippet from my book CHRISTIAN HOPE THROUGH FULFILLED PROPHECY:
Numbers in the Bible are often symbolic, especially in apocalyptic texts. The number seven in the Bible is symbolic for completeness. Even numbers (as opposed to odd numbers) have their own symbolism. The number ten in the Bible is symbolic for manyness. The thousand years is a symbolic term for many years. The Greek word translated “thousand” is chilioi. According to the online Blue Letter Bible, the root word of chilioi means “plural of uncertain affinity.”
As put by Steve Gregg, “The number ‘a thousand’ is frequently used in Scripture without the intention of conveying statistical information. It is given as the number of generations to which God keeps His covenants (Deuteronomy 7:9), the number of hills upon which God owns the cattle (Psalm 50:10), the number of enemy troops that one Israelite shall chase (Joshua 23:10), the number of those who shall fall ‘at your side’ as opposed to the ten thousand who will fall at ‘your right hand’ (Psalm 91:7), etc. Furthermore, the expression ‘a thousand years’ is never used elsewhere in Scripture for an actual number of years, but only to suggest the idea of a very long time (cf. Psalm 90:4; Ecclesiastes 6:6; 2 Peter 3:8).”
The usage of a thousand years is not necessarily a long period of time, as it can also be understood symbolically in context to mean “completeness” or “fullness” (Psalm 50:10-12). According to Anthony A. Hoekema, “The number ‘thousand’ which is used here must not be interpreted in a literal [straight forward] sense. Since the number ten signifies completeness, and since a thousand is ten to the third power, we may think of the expression ‘a thousand years’ as standing for a complete period. . . . of indeterminate length. . . . We may conclude that this thousand-year period extends from Christ’s first coming to just before his Second Coming.”
I am persuaded that the millennium is a figurative period of time between the cross and Parousia. It was about 40 years culminating in the destruction of Jerusalem and the temple in AD 70, when Jesus “came” in judgment against Old Covenant Israel. This is a remarkable parallel to the 40-year period before entering the LITERAL promised land after leaving Egypt. In the first century, there was a 40-year period of trials and tribulation until entering the SPIRITUAL promise land — the New Heavens and New Earth — when Old Covenant Judaism came to a final end.
There is more evidence that the millennium came to a close in AD 70:
1. The wedding of the Lamb (Revelation 19:7; 21:9) was fulfilled at the destruction of Jerusalem in AD 70 per Matthew 22:1-14.
2. The Great White Throne Judgment (Revelation 20:11-15) was fulfilled at the end of the thousand years (Revelation 20:1-6) at the defeat of Babylon―who is clearly identified as Jerusalem (Revelation 11:8)―thus at Jerusalem’s fall in AD 70.
3. The New Heaven and New Earth (Revelation 21:1-8; Isaiah 65-66) came after the millennium. The New Heaven and New Earth is the New Covenant order, which came to its fullest expression when the Old Covenant order was ushered out in AD 70 (Hebrews 8:13). See the articles here: https://prophecyquestions.com/category/revelation-2.
4. Revelation is about things that “must shortly take place” (Revelation 1:1; 22:6) for the “time is near” (Revelation 1:3; 22:10).
5. Some Jewish rabbis thought that the millennium would be 40 years in duration.
6. Of the some two dozen mentions of the Second Coming in the New Testament, all but one (Acts 1:9-11) either specifically or by implication would be while some living in the first century were still alive (example: Matthew 16:27-28; etc.).
We do not wait for a millennial kingdom of Christ. It was delivered to believers in the first century (Colossians 1:13).