Q: Revelation 21:27 states that nothing unclean shall enter heaven. It seems that this is speaking about the “New Heaven.” What about heaven in our present time, will anything unclean enter it?
A: No, nothing unclean can enter heaven, even today. The Catechism of the Catholic Church explains:
1024 This perfect life with the Most Holy Trinity — this communion of life and love with the Trinity, with the Virgin Mary, the angels and all the blessed — is called “heaven.” Heaven is the ultimate end and fulfillment of the deepest human longings, the state of supreme, definitive happiness.
Obviously, we cannot be definitively and supremely happy while we remain unclean, meaning that we must be thoroughly morally cleansed so that we no longer have sinful desires and no longer sin, so that we will be only good. Only then can we be finally and wholly happy and perfectly united with God. Heaven is, as the Catechism states, “perfect life with the Most Holy Trinity.”
Thus we must be purified of any imperfection in us that remains at the point of death:
1054 Those who die in God’s grace and friendship imperfectly purified, although they are assured of their eternal salvation, undergo a purification after death, so as to achieve the holiness necessary to enter the joy of God.
What Revelation 21 describes is the change in the location of the community of the blessed. At the present time, heaven is located in the current heaven. At the end of Revelation we are told that it (or at least the human portion of it) will move to the New Earth. We read:
Then I saw a new heaven and a new earth; for the first heaven and the first earth had passed away, and the sea was no more. And I saw the holy city, New Jerusalem, coming down out of heaven from God, prepared as a bride adorned for her husband; and I heard a loud voice from the throne saying, “Behold, the dwelling of God is with men. He will dwell with them, and they shall be his people, and God himself will be with them; he will wipe away every tear from their eyes, and death shall be no more, neither shall there be mourning nor crying nor pain any more, for the former things have passed away” (Revelation 21:1-4, RSV).
Heaven-the state of being perfectly united with God–is pictured as the city New Jerusalem, which at present is in the current heaven, but when the world is regenerated and made new, the New Jerusalem will “come[e] down out of heaven” to the New Earth, so that “the dwelling of God [will be] with men.” We will then, literally and truly, experience heaven on earth.
Revelation 21:27 tells us that “nothing unclean shall enter it,” but this is as true of the New Jerusalem now as it will be then, since the essence of heaven is the same now as then–perfect life in union with God, the saints, and angels.