What does Revelation 22:15 mean?

Whereas the righteous may enter New Jerusalem, the unrighteous are barred from entering the city. They are described as dogs and sorcerers, sexually immoral and murderers, idolaters, and lovers and doers of falsehood.

"Dogs" was an insulting term the Judaizers applied to the Gentiles. In the first century, packs of wild dogs roamed through city streets and scavenged from trash and dead bodies. They were considered unclean. "Sorcerers" were those who practiced black magic, with an emphasis on drugs and potions. The sexually immoral committed adultery and fornication. Murderers snuffed out the lives of others. They disregarded the sanctity of human life and the fact that human beings bear the image of God.

Idolaters worship false gods, including material objects and even human beings. Those who bore the mark of the beast were idolaters because they worshiped the beast. Colossians 3:5 cites covetousness as idolatry, thereby identifying as idolaters those who worship money and what it can buy. Liars, too, are excluded from the eternal city.

The point of this verse is not to make an exhaustive list, or to suggest that "only" these sins are considered. The point is that those who persist in those types of behaviors—who reject Christ and refuse to repent—are going to find themselves separated from God in eternity. It's also not a claim that such sins are unforgivable; Paul recalled that some of the Corinthians had practiced such abominable sins before God saved them (1 Corinthians 6:9–11).