Jamieson, Fausset and Brown on Revelation 21:27

27. anything that defileth--Greek, "koinoun." A and B read [koinon,] "anything unclean."

in the Lamb's book of life--(See on Re 20:12; Re 20:15). As all the filth of the old Jerusalem was carried outside the walls and burnt there, so nothing defiled shall enter the heavenly city, but be burnt outside (compare Re 22:15). It is striking that the apostle of love, who shows us the glories of the heavenly city, is he also who speaks most plainly of the terrors of hell. On Re 21:26, 27, Alford writes a Note, rash in speculation, about the heathen nations, above what is written, and not at all required by the sacred text: compare Note, see on Re 21:26.