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I would say that the first reason would be that the new Jerusalem will be of an enormous size that will accommodate all the redeemed from throughout history -- a square of 12,000 stadia (approximately 1,400 miles) on each side. The second reason would be that all the inhabitants of the city will have the sanctification that was begun within them when they accepted the forgiveness available only through faith in Christ and received the indwelling Holy Spirit, completed and at its full fruition for eternity, living in the harmony and love that God intended for humanity at creation. The third reason would be the abiding, eternal, visible presence of God Himself (the glorified Christ) in the city. The fourth reason would be that those outside the city's walls will be only the unredeemed who were unwilling to seek the forgiveness of their multitude of sins that was offered through faith in Christ. (Revelation 21:8 describes those individuals and their sins as "the cowardly [i.e., those who were deterred from faith by fear of the persecution that they would face], the unbelieving, the vile, the murderers, the sexually immoral, those who practice magic arts, the idolaters, and all liars.") (See the further elaboration on this that can be found at https://biblehub.com/revelation/21-8.htm) (Revelation 21 describes their ultimate fate as consignment to the fiery lake of burning sulfur, which is called "the second death", and which (unlike the first death) will last for eternity.)
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