Verse 27 does not imply that there will be sin on the New Earth to endanger the City, but to show that the City will never be contaminated by evil of any kind.
Outside the walls of this beautiful City, spread over the surface of the New Earth, nations shall dwell, whose kings shall bring their glory and honour into it, but nothing that will defile or work abomination shall ever enter in through those Gates of Pearl, for there will be no sin on that New Earth (21:24-27).
Who are to be the happy inhabitants of this New Earth? Where did the people who inhabited the earth after the Flood come from? They were the lineal descendants of Noah. How did they escape the Flood? They were saved in an ark which God provided (Gen. 6:13-16). It may be then that God, during the renovation of the Earth by fire, in some manner, not as yet revealed, will take off righteous representatives of the Millennial nations that He purposes to save, and when the earth is again fit to be the abode of men, place them back on the New Earth, that they may increase and multiply and replenish it, as Adam (Gen.1:27, 28), and Noah (Gen.9:1), were told to multiply and replenish the present earth.
If God could take off Elijah for the purpose of sending him back again to herald the Second Coming of the Lord, surely God can take off representative men (and women) from the nations and put them back again on the New Earth to repopulate it. If this is not God's plan then we have one type in the Scriptures that has no antitype, for Noah's Ark, which is a type, has no antitype unless it be this.
It is clear from the Scriptures that God does not purpose to create a new race for the New Earth. His promise as to Israel is that the descendants of Abraham shall inherit this earth for a "thousand generations", or 33,000 years; now this is not possible unless they are transplanted to the New Earth. And this is just what God has promised.
"For as the New Heavens and the New Earth, which I will make, shall remain before Me, saith the Lord, so shall your seed and your name remain" ( Isa.66:22). It seems clear from the presence of the Tree of Life in the Garden of Eden, that God intended the human race to populate the Earth, and when it became too thickly populated, to use the surplus population to colonise other spheres. Our "Solar System" is only in its infancy.
The Earth is the only one of its planets as yet habitable. Where are the inhabitants for the other planets to come from? Think you that the planets of our Solar System, and the planets of other solar systems, of which the stars are the suns, were made simply to adorn the heavens for our little earth? God does not plan things on a small scale, and it magnifies His power and wisdom to believe that He created man in His own likeness, a created being higher than the angels, and gifted with the power of reproduction, that He might by means of him populate the Universe. This magnifies the Scheme of Redemption.
Think you that God gave His Son to die on Calvary just to redeem a few millions of the human race? Why He could have blotted them out, as He probably did the pre-Adamite race, and created a new race, and Satan would have laughed because he had the second time blocked God's plan for the peopling of this earth. No, God will not permit Satan to block His plan for peopling this earth with a sinless human race. The death of Christ was not merely to redeem a few millions of the human race, but to redeem the earth, and the race itself from the curse of sin, and the dominion of Satan.
The Apostle James tells us that we are only the first fruits of His creatures (James 1: 18). What then must the harvest be? The Universe is young yet. We are only in the beginning of things, for "Of the increase of His government and peace there shall be no end" (Isa.9:7). When this Earth shall have gone through its baptism of fire, and shall be again fit for the occupancy of man, the representatives of the saved nations (21:24) will be men and women in whom no taint of sin will remain, and who cannot therefore impart it to their offspring, who will be like the offspring of Adam and Eve would have been if they had not sinned. This magnifies the whole scheme of redemption, and justifies God in the creation of the human race.
The New Nations
24 And the nations of those who are saved shall walk in its light, and the kings of the earth bring their glory and honor into it.
25 Its gates shall not be shut at all by day (there shall be no night there).
26 And they shall bring the glory and the honor of the nations into it.
27 But there shall by no means enter it anything that defiles, or causes an abomination or a lie, but only those who are written in the Lamb's Book of Life.
Rev 21:24-27 (NKJV)