Revelation 21:7-8
7 He that overcometh shall inherit all things; and I will be his God, and he shall be my son.
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.
Charles Bing - Grace, Salvation, and Discipleship - Sinners and the Second Death, Revelation 21:8
(Salvation) Interpretation: Gross sinners will go to hell, including those who profess to be Christians but commit these sins or those Christians who commit these sins and lose their salvation.
(Discipleship) Interpretation: In contrast to the believers and faithful overcoming Christians in God's presence, these unbelieving sinners will perish in the lake of fire.
There is no question about the fate of those mentioned in this passage - they are all consigned to eternal punishment in the lake of fire. We have read previously that this lake is the destiny of all unsaved people. However, this passage makes it appear that only gross sinners perish there or that their sins are the reason for their punishment. Some would also argue that believers who practice such gross sins lose their salvation and suffer the fate of the lake of fire.
An honest reading of the passage would have to admit that not all the sins listed are of the most severe kind. The list includes the "cowardly" (deilos, means timid or fearful) and "liars." Certainly this broadens the application to most or all Christians, so it cannot be these specific sins that prove one is unsaved or can lose his or her salvation.
The point of mentioning unbelievers and their sins in this passage is to contrast (v. 8 begins with "But") their fate with that of all believers. The text implies that there are those believers who have received the water of eternal life freely (v. 6) and that some of those believers are rewarded for their faithfulness ("overcomers," v. 7). The fates of these three groups differentiate between the importance of Jesus' free offer of eternal life to all in verse 6, the reward earned by some for serving Him in verse 7, and the consequence of rejecting Him in verse 8. The contrast between rewarded believers and unbelievers is also found in Revelation 22:11-15.
To anyone who reads these final words of Revelation, it is clear that there are only two fates for all mankind, a blessed eternity in God's presence or a horrible eternity shut out from His presence. But these final words also include the way to a blessed eternity (Revelation 21:6, 22:17). Eternal life is a free gift. Receiving it is the only way to avoid the lake of fire.