‘And under no circumstances will anything unclean enter it, or he who makes an abomination and a lie, but only those who are written in the Lamb's book of life.'
This final sentence confirms all we have said. The city only consists of those whose names are written in the Lamb's book of life. It is only for the true people of God. No ‘living nations' can enter it if they are not of the people of God, for it is for the people of God and for them alone. Totally excluded (because they have been dealt with elsewhere) are idolaters, those who are unclean, which probably especially refers to sexual uncleanness in accordance with what we have seen earlier in the book, but also includes uncleanness of any sort, and any who have preferred falsehood to truth. The city of God is for ‘virgins', for those who are without blemish, for those in whose mouth there is no lie (Revelation 14:4-5). Only the perfected can make up the city of God.
The idea, of course, is that those who do such things are excluded in principle. They are not outside trying to sneak in. They no longer exist. Their exclusion was settled long before.
It is clear that no such city would exist in its physical proportions and make up. But it is not intended that it should exist physically (even if anything could be seen as physical in the new world). What is intended is to bring out the splendour, the glory, the privilege, the perfection and the fullness of the people of God.