"The catalog of sins [in Revelation 21:8*] concludes with all liars. The phrase likely points to a focus on those whose Christian profession is betrayed either by compromising behavior or false doctrine. The word refers to false apostles in 2:2 and to ethnic Jews falsely claiming to be God’s true people in 3:9. John uses the phrase elsewhere to refer to those within the church whose behavior or doctrine contradicts their purported profession of faith in Jesus (1 John 2:4, 22; 4:20; 5:10).--------------------------------------/
"An almost identical list of sinners as in v. 8 occurs in 22:15, and a similar but abbreviated catalog concludes ch. 21 (v. 27). Both these subsequent lists end with “lying,” which emphasizes the judgment on those whose apparent Christian profession is contradicted by their lifestyle or false doctrine. Tit. 1:16 expresses a similar thought: “they profess to know God, but by their deeds they deny Him, being detestable and disobedient, and worthless for any good deed.” Possibly also in mind are those who promote the lie about the beast (13:12-15). In 3:9, lying is associated with being a devotee of Satan (likewise in 2:9), the original liar (John 8:44) and deceiver (Rev. 12:9; 20:2-3). Rev. 14:5 says the genuine people of God persist in not lying, which refers to perseverance in professing the truth about the gospel and in not compromising. The absence of liars in the new world shows that the coming order will exist on a higher moral level than even the pre-fall cosmos, where the Satanic liar was allowed entrance."**
* New Testament | Revelation 21:8
8 But the fearful, and unbelieving, and the abominable, and murderers, and whoremongers, and sorcerers, and idolaters, and all liars, shall have their part in the lake which burneth with fire and brimstone: which is the second death (bold emphasis added.)
**Beale, G. K.; Campbell, David. Revelation: A Shorter Commentary (pp. 473-474). Wm. B. Eerdmans Publishing Co.. Kindle Edition.