Jesus' prophetic Second Coming will usher in a time of unparalleled peace and prosperity for both Jerusalem and the earth. Because Satan's brief rule of mankind brought destruction to much of the planet, however, a new city will need to be built. This New Jerusalem will be the headquarters from which Christ and his saints rule humanity for 1,000 years (Revelation 20:4, 6).
The devil, at the end of the Millennium, will be allowed to foster another rebellion against God. He will deceive as many people as he can and lead them in a climactic assault on Jerusalem (Revelation 20:7 - 9). His efforts, however, will fail miserably. Fire will come down from the Eternal and devour both him and his army. Why, however, is yet another Jerusalem needed?
God, after the above events, permanently destroys his adversary and judges the rest of humanity who were not in the first resurrection (Revelation 20:10 - 15). He will then, through fire, remove all traces of evil from the universe (2Peter 3:7, 12) and create a new heaven and earth (Revelation 21:1). A distinctly unique Jerusalem, alluded to in many Scriptures (John 14:2 - 3, Hebrews 11:10, 16, etc.), will then be made directly by the hand of the Almighty (Revelation 21:2)!
Why is God recreating the entire physical universe including the earth? It is because he will personally dwell with all those who accepted his offer of salvation and who are now spirit beings! His awesome plan to instill holy, righteous character into as many humans as are willing will have completed.
The New Jerusalem created by God will be a perfect cube, with its length, width and height being 12,000 furlongs (Revelation 21:16). This translates, using modern measurements, into a city that is 1,500 miles (2,414 kilometers) square. It will have a total area of roughly 3.37 billion cubic miles (14 billion cubic kilometers). How does this amazing Jerusalem compare to the current size of the earth? The earth's total surface area is about 197 million square miles (510 million square kilometers). The eternal city will be many times bigger than this!
The walls of the newly made Jerusalem will be 144 cubits thick (Revelation 21:17). Bible commentaries vary regarding the length of a cubit, primarily because it was traditionally the distance between a man's elbow and the tip of his middle finger. Given a range of between 17 to 21 inches (43 to 53 centimeters), the walls will be anywhere from 204 to 252 feet thick (or 62 to 77 meters)!
The New Jerusalem will be the center of both the physical and spiritual universes. God will move his and Jesus' throne to the beautiful city that he will personally decorate "as a bride adorned for her husband" (Revelation 21:2, HBFV). It is then that he will eternally enjoy the fruit of his labors to make spiritual beings with the same righteous character he possesses!