What does Revelation 21:27 mean?

Although New Jerusalem's gates are always open, not a single trace of evil can enter the city. No uncleanness can pass through the gates and mar the city's pure, spotlessly clean environment. What a joyful expectation this revelation must have brought to the apostle John. Many of the cities he had visited in the Mediterranean region allowed their streets to become garbage dumps and open sewers. The back alleys could be described as nothing short of smelly and foul. Ceremonial uncleanness was intended to symbolize moral filth; New Jerusalem will be gloriously free of both.

The New Jerusalem is inaccessible to criminals, liars, and idolaters. Those who practice fraud and deception are banished forever from God's beautiful, dazzling city. In fact, as explained in prior verses, all sin and evil has been entirely defeated and banished to hell by this point in history (Revelation 20:10–15). As Revelation 21:8 points out, the cowardly, faithless, detestable, murderers, sexually immoral, sorcerers, idolaters, and liars will spend eternity in the lake of fire. Only those whose names are recorded in the Lamb's book of life are welcome in the New Jerusalem. Of all the references to the book of life in Revelation (Revelation 3:5; 13:8; 17:8; 20:12, 15; 21:27), 21:27 is the only one that calls the book of life, "the Lamb's book of life."