And I saw the holy city, new Jerusalem, coming down out of heaven from God, prepared as a bride adorned for her husband.

Revelation 21:2

Yesterday, I saw that the New Heavens and New Earth are the Kingdom--this age. Today, I want to look at what that makes the new Jerusalem.
In just a few verses, that becomes clear:

Then came one of the seven angels who had the seven bowls full of the seven last plagues and spoke to me, saying, “Come, I will show you the Bride, the wife of the Lamb.” And he carried me away in the Spirit to a great, high mountain, and showed me the holy city Jerusalem coming down out of heaven from God...

Revelation 21:9-10

The new Jerusalem is the Church. So, we are in the new Jerusalem now. Huh? Check this out:

But you have come to Mount Zion and to the city of the living God, the heavenly Jerusalem, and to innumerable angels in festal gathering, and to the assembly of the firstborn who are enrolled in heaven, and to God, the judge of all, and to the spirits of the righteous made perfect...

Hebrews 12:22-23

The Jerusalem above is free, and she is our mother.

Galatians 4:26

The one who conquers, I will make him a pillar in the temple of my God. Never shall he go out of it, and I will write on him the name of my God, and the name of the city of my God, the new Jerusalem, which comes down from my God out of heaven, and my own new name.

Revelation 3:12

Why is it coming down from heaven? Note that the phrase born again in John 3:3 and 3:7 can also be translated born from above. (See the footnotes in your Bible). Ephesians 2:19 says we "are no longer strangers and aliens, but [we] are fellow citizens with the saints and members of the household of God." Philippians 3:20 says "our citizenship is in heaven."
John goes on to record a loud voice saying:

“Behold, the dwelling place of God is with man. He will dwell with them, and they will be his people, and God himself will be with them as their God."

Revelation 21:3

Notice how Paul also connected these ideas:

So then you are no longer strangers and aliens, but you are fellow citizens with the saints and members of the household of God, built on the foundation of the apostles and prophets, Christ Jesus himself being the cornerstone, in whom the whole structure, being joined together, grows into a holy temple in the Lord. In him you also are being built together into a dwelling place for God by the Spirit.

Ephesians 2:19-22

Just as God promised in Leviticus 26:11, "I will make my dwelling among you..." and 2 Corinthians 6:16, "we are the temple of the living God..."
We, the Church, are the new Jerusalem.