Christians generally believe that heaven is sinless. Therefore, if the New Jerusalem of heaven comes down to earth in Rev 21:2, this would then imply that the earth would become sinless after the end of the age as well. And yet this cannot be true as the Bible explicitly says that there is sin in heaven: “For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of EVIL in the HEAVENLY REALMS.” (Eph 6:12.) Similarly, God is said to punish the host of heaven for their sin according to Isaiah 24:21: “It shall come to pass in that day that the Lord will punish on high the host of heaven, and on earth the kings of the earth.”
Sin is a direct and seemingly inevitable consequence of free will. If free will continues in heaven then sin and wickedness would seemingly inevitably follow. (Though it should be noted that free will violates the law of cause and effect and as such is generally believed by philosophers to be a stubborn illusion. And if free will does not actually exist as most philosophers would assert, a sinless state in heaven becomes far more likely. See Because of the Paradox Within the Law of Cause and Effect, This Law Does Not Disprove Free Will.1)
The Temple in Jerusalem like all Temples of the ancient world was believed to be a junction between heaven and earth (Psalm 78:69, Hebrews 8:5; 9:24-26). This is why the New Jerusalem is described in Temple imagery (Rev 21:3, 16, 22; 22:1-2) like the Holy of Holies which is a symbol of heaven (Hebrews 8:5; 9:24-26). The New Jerusalem is described in Temple trappings because the New Jerusalem is the Church, the new Temple of God (2 Cor 6:16). And because the Church receives forgiveness through faith in Christ, it is through this forgiveness granted to the Church that the New Jerusalem remains sinless—not through human effort, but through grace.