READER>> How can you say Revelation 21:8 does not apply to humans? If you say that only fallen angels are sentenced to the Lake of Fire, since when were demons cowardly, unbelieving, vile, murderers, sexually immoral, magicians, idolaters, and liars? This is clearly speaking about human unbelievers.
ME>> This is a most excellent question.
Here is Revelation 21:8 (WEB) again,
But for the cowardly, unbelieving, sinners, abominable, murderers, sexually immoral, sorcerers, idolaters, and all liars, their part is in the lake that burns with fire and sulfur, which is the second death.
The demonstration of Grace is God's purpose in creation and redemptive history and so grace should always be the interpretive lens of Scripture. The believing elect, having received grace and being led by the Holy Spirit, are well equipped to interpret the Scripture. The Holy Spirit even says boldly that, "you don’t need for anyone to teach you," for we have an anointing from Him, I John 2:27 (WEB).
To that end I make an observation, the passage says that "all liars, their part is in the lake that burns with fire and sulfur." I boldly assert through the interpretive lens of grace that in this case "all" does not mean "all" in this context. The immediate context and the whole context of Scripture is essential for rightly dividing the word of truth. In my key exegesis of Romans 11:32, I explained that "all" does mean all because all mankind are sinners. Reread my article, Romans 11:32-36 for my complete argument. However, in the case of Revelation 20:8 "all" cannot mean all liars, because everyone is a liar according to Romans 3:4. If all liars are damned to the Lake of Fire then you and I are damned as well. Likewise even Christians are at times cowardly, unbelieving, vile, murderers, sexually immoral, magicians, and idolaters. Yet these sins are forgiven for all mankind through the death of Christ. No doubt even your theology agrees with my logic so far. So in light of all of Scripture, in this case "all" must mean all those outside of Christ, that is the non-elect. Only the non-elect sinners are consigned to the Lake of Fire. However, those predestined for grace certainly are saved from the Lake of Fire even though we are liars and sinners of various types. Do you follow this logic?
So the damned are those whose sins are not paid for, those who owe an obligation to the judge at the Great White Throne Judgment, but have nothing to pay the obligation. However, the sins of all mankind are paid for, according to I John 2:1-2. Thus the only beings remaining with a sin debt are the fallen angels. Coincidentally only fallen angels are explicitly named as cast into the Lake of Fire, that is Satan, The Beast, and The False Prophet. Furthermore, the Scriptures explicitly state that the Lake of Fire is "prepared for... the Devil and his angels" (Matthew 25:41 WEB).
But can Revelation 21:8 really only apply to angels? Yes, I've already proved that God's chosen people are also guilty of all these sins, but exempt from damnation due to the fact that our sins are paid! That is the good news!! Fallen angels, however, are vile, but their vileness is not atoned. Jesus became a man and is the vicarious substitute for mankind, but not the angel kind. Consider that Satan is called a murderer and the Father of Lies, John 8:44. Ezekiel 28 also highlights Satan's idolatry. He worshiped his own beauty and not the beauty of Christ. Yet can we say that angels are cowardly, sexually immoral, and magicians? This seems odd to give these labels to the fallen angels. Yet Satan and his demons are guilty of these crimes even if only because they tempt and applaud mankind's stumbling in these areas. Consider also that some believe that embodied fallen angels are specifically guilty of sexual immorality in Genesis 6:1-4 and were punished as explained in 2 Peter 2:4. Though I am uncertain of these details, I am certain that fallen angels will be condemned at the Great White Throne Judgment as explained in Daniel 12:1-4, Matthew 25:31-46, and Revelation 20:11-15. In the end, Satan and his demonic following are the only people outside of Christ at the Great White Throne Judgment as they are sentenced the Lake of Fire for The Ages of The Ages. Finally, Isaiah believed that one day all mankind would bow down in worship before the Lord, Isaiah 66:22-24.
Friend, is your question really a question of Biblical exegesis? Or instead is your question really about how grace is applied to the forgiveness of your own sins for your own salvation? Are you convinced that your choice of faith has signed the deal for your salvation? Why not trade in your "personal salvation" to join the salvation of ALL God's people? Why not trust that Christ signed the deal for your salvation with his blood at Calvary, as well as the salvation of ALL mankind.