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Rev 21:8 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.
The word translated fearful is used just 3 times in the New Testament.On the other two occasions it is a fear associated with a lack of faith.
Matthew 8:26 And he saith unto them, Why are ye fearful, O ye of little faith? Then he arose, and rebuked the winds and the sea; and there was a great calm.
Mark 4:40 And he said unto them, Why are ye so fearful? how is it that ye have no faith?
I don’t think it refers to those in fear at Christ’s return as described in Luke 21.
Luke 21:26 Men‘s hearts failing them for fear, and for looking after those things which are coming on the earth: for the powers of heaven shall be shaken.
Revelation 21 is speaking of the final judgment at the end of the millennial reign of Christ, so the fear it describes must be in those who are subject to judgment.This means that they understand the purpose of God, have refused his salvation and live in fear of the judgment.The writer to the Hebrews speaks of those who having known the truth fall away.
Hebrews 10:26 For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins,27 But a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries.28 He that despised Moses’ law died without mercy under two or three witnesses:29 Of how much sorer punishment, suppose ye, shall he be thought worthy, who hath trodden under foot the Son of God, and hath counted the blood of the covenant, wherewith he was sanctified, an unholy thing, and hath done despite unto the Spirit of grace?30 For we know him that hath said, Vengeance belongeth unto me, I will recompense, saith the Lord. And again, The Lord shall judge his people.31 It is a fearful thing to fall into the hands of the living God.
The same prospect of Judgment caused Felix to tremble.
Acts 24:25 And as he (Paul) reasoned of righteousness, temperance, and judgment to come, Felix trembled...
Not all who experience fear at Christ’s return will be responsible for judgment, and some of those in fear who survive may turn to Christ and be accepted at the judgment.For this reason I think that the fear must be linked to knowing the purpose of God and refusing to obey him.
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I hope this helps.