What does Revelation 21:10 mean?

John reports in this verse that the angel carried him away to a high mountain, where he saw the holy city of Jerusalem descending from heaven from God. John was in the Spirit, when this occurred.

A sharp contrast exists between historic Jerusalem and the New Jerusalem of the millennium and eternity. The former Jerusalem is often disrupted by internal and external strife. The New Jerusalem will be a city of perfect peace. Men designed and built the former Jerusalem. God is the designer and builder of the New Jerusalem. The Babylonians, Romans, and centuries later, the Gentiles of the tribulation overran the former Jerusalem. The New Jerusalem is secure, indestructible, and eternal.

The current, earthly Jerusalem experiences pain, suffering, death, and mourning. Sorrow, pain, suffering, and death are absent from the New Jerusalem (Revelation 21:4). Several idolatrous, unrighteous kings ruled from the former Jerusalem. The kings who enter the New Jerusalem are righteous worshipers of God (Revelation 21:24–27) and the New Jerusalem is the capital from which the King of kings and Lord of lords rules. The former Jerusalem stoned God's prophets (Matthew 23:37). The New Jerusalem is a safe home to His prophets. The former Jerusalem clamored for Christ's crucifixion (Matthew 27:22–23). In the New Jerusalem Christ doesn't bear a cross; He wears a crown.