The New Jerusalem corresponds to the New Covenant. The Old Jerusalem corresponded to the Old Covenant that existed at the time of Paul before 70AD.
GALATIANS 4 “Tell me, you who want to be under the LAW, are you not aware of what the law says? 22 For it is written that Abraham had two sons, one by the slave woman and the other by the free woman. 23 His son by the slave woman was born according to the flesh, but his son by the free woman was born as the result of a divine promise. 24 These things are being taken figuratively: The women represent TWO COVENANTS. One covenant is from Mount Sinai and bears children who are to be slaves: This is Hagar. 25 Now Hagar stands for Mount Sinai in Arabia and corresponds to the PRESENT CITY of JERUSALEM, because she is in slavery with her children. 26 But the Jerusalem that is above is free, and she is our mother.”
HEBREWS 12 “But YOU HAVE COME to Mount Zion, to the city of the living God, the HEAVENLY JERUSALEM. You have come to thousands upon thousands of angels in joyful assembly, 23 to THE CHURCH of the firstborn, whose names are written in heaven. You have come to God, the Judge of all, to the spirits of the righteous made perfect, 24 to Jesus the mediator of a NEW COVENANT, and to the sprinkled blood that speaks a better word than the blood of Abel.
REVELATION 21 “Then I saw “a new heaven and a new earth,” for the first heaven and the first earth had passed away, and there was no longer any sea. 2 I saw the Holy City, the new Jerusalem, coming down out of heaven from God, prepared as a bride beautifully dressed for her husband. 3 And I heard a loud voice from the throne saying, “Look! God’s dwelling place is now among the people, and he will dwell with them”
The Old Jerusalem was a harlot who was unfaithful, but the New Jerusalem is spotless and sinless because Christ has washed Her in His faithfulness. (Ephesians 5)
Under the Old Covenant, Gentiles were not allowed into the Temple! But in the New covenant, this is a spiritual truth that there is no Jew or Gentile and there is no separation between the nations anymore. We are all One in Christ. The New Jerusalem is spiritual. It is not a literal physical city. But it is you and me – the presence of God is within us.
Old things have passed away (the Old covenant, Old Jerusalem, Law, wrath, Temple, judgment!)
All things are NEW (New covenant, New Jerusalem, Spirit, Christ is with you in Spirit)