"Then I saw a new heaven and a new earth, for the first heaven and the first earth had passed away, and the sea was no more. And I saw the holy city, new Jerusalem, coming down out of heaven from God, prepared as a bride adorned for her husband." Revelation 21:1-2 (ESV)
What - or who is the New Jerusalem? If she is not the church - then who is she? That one prepared as a bride adorned for her Husband....? I take a postmillennial view in the sense I see the church as being the heart of the Kingdom of God and the gospel spreading to and throughout the nations much like the way yeast infiltrates a lump of dough. Take note: this does not come about by the power of man like some magical "utopia" but happens by the power of God through His Spirit.
Like blood flowing in and throughout the body, so the church is the heart of the kingdom. Granted, the new heavens and new earth came in principle with the incarnation/cross/resurrection of Christ but the influence is spreading (coming down) like leaven in the loaf. As C.S. Lewis said, aliens have invaded enemy occupied territory and the results are nothing less than world domination.
That conquering message of the gospel of the love God through Jesus Christ has been spreading for 2000 years now and the outworking of it cannot be stopped. Christ said he will build his church and the gates of hell will not prevail against her. How is she built? Through the proclamation of the gospel which is the foundation established by the prophets and the Apostles - Christ being the chief Cornerstone.
Jesus used the metaphor of a mustard seed in Luke 13 and in Matt. 13, "It is like a grain of mustard seed that a man took and sowed in his garden, and it grew and became a tree, and the birds of the air made nests in its branches.”
The Kingdom is the new Jerusalem, coming down and infiltrating the entire world until the knowledge of the Lord covers the earth like the water covers the sea. It's that stone cut from the mountain by no human hand, which although small, wound up destroying all other kingdoms, mentioned in Daniel chapter 2. It's the proclamation of the Kingdom of Christ - the gospel. It's the entire theme of God's Word and history! The church, the new Jerusalem, as I said before, she is the heart of the kingdom and through her the kingdom spreads by word and sacrament.
That spread of yeast through the bread, or the bride coming down from heaven, in one sense has only just begun. In other words, I tend to see us as being still in the very early stages of church history - Acts Chapter 29 or possibly 30, if you will. Again, I'm not dogmatic on this, but this explains a bit of my understanding of that passage in Revelation 21.