You have misread the verse. It says he comes in flaming fire to exact revenge, not that he uses flaming fire to exact revenge. Rev 19 says he uses a sword not fire.
The flaming fire is a reference to the lightning speed of his return:
2Th 1:7 And to you who are troubled rest with us, when the Lord Jesus shall be revealed from heaven with his mighty angels,
2Th 1:8 In flaming fire taking vengeance on them that know not God, and that obey not the gospel of our Lord Jesus Christ:
In flaming fire is regarding Christ not the unsaved. Flaming means a flash and fire is specifically lightning according to the Strongs. So Christ comes In a flash of lightning to arrive and take vengeance (the sword of his mouth Rev 19) on the unsaved and that shouldn't be surprising because we were told that before:
Mat 24:27 For as the lightning cometh out of the east, and shineth even unto the west; so shall also the coming of the Son of man be.
This denotes the speed of his return like a flash of lightning. It has nothing to do with using fire or lightning against the unsaved. To think that is to misread the verses.
Just read Rev 19 where the battle is described and you will see ZERO fire or lightning used to kill anyone.
2Th 1:8 In A FLASH OF LIGHTNING taking vengeance on them that know not God, and that obey not the gospel of our Lord Jesus Christ:
2Th 1:9 Who shall be punished with everlasting destruction from the presence of the Lord, and from the glory of his power;
While this is the very next verse it is wrong to insert that this is "at the same time" as the previous verse. The return of Christ and the vengeance against those that know not God happens a long time before the "everlasting destruction".
2Th 1:10 When he shall come to be glorified in his saints, and to be admired in all them that believe (because our testimony among you was believed) in that day.
And this goes back to the second coming. If you think "in that day" means all these events happen on the same literal day you are misunderstanding the verses and we know that because of what Rev 19-20 present in detail.
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He comes so fast and brightly the use of lightning is used. He doesn't send down lightning bolts or anything. He comes in flaming fire not casts out flaming fire. The same exact thing is here:
Mat 24:27 For as the lightning cometh out of the east, and shineth even unto the west; so shall also the coming of the Son of man be.
It's about the speed because lightning was the fastest thing man knew of in those days. This essentially is "light speed" and is what this verse is talking about:
Barnes:
In flaming fire - This is a circumstance which is not noticed in the account of his appearing in the parallel place in 1Th_4:16. The object of the apostle here seems to be to represent him as coming amidst vivid flashes of lightning. He is commonly described as coming in clouds, and to that common description there is here added the image of incessant lightnings, as if the whole heavens were illuminated with a continued blaze.
2 Thessalonians 1:8
(CJB) in a fiery flame. Then he will punish those who don't know God, that is, those who don't listen to the Good News of our Lord Yeshua and obey it.
(ESV) in flaming fire, inflicting vengeance on those who do not know God and on those who do not obey the gospel of our Lord Jesus.
(Geneva) In flaming fire, rendring vengeance vnto them, that doe not know God, and which obey not vnto the Gospel of our Lord Iesus Christ,
(Greek NT TR) εν πυρι φλογος διδοντος εκδικησιν τοις μη ειδοσιν θεον και τοις μη υπακουουσιν τω ευαγγελιω του κυριου ημων ιησου χριστου
(GW) He will take revenge on those who refuse to acknowledge God and on those who refuse to respond to the Good News about our Lord Jesus.
(LITV-TSP) in flaming fire giving full vengeance to those not knowing God, and to those not obeying the gospel of our Lord Jesus Christ, Isa. 66:15; Jer. 10:25
(KJV) In flaming fire taking vengeance on them that know not God, and that obey not the gospel of our Lord Jesus Christ:
(KJV+) InG1722 flamingG5395 fireG4442 takingG1325 vengeanceG1557 on them that knowG1492 notG3361 God,G2316 andG2532 that obeyG5219 notG3361 theG3588 gospelG2098 of ourG2257 LordG2962 JesusG2424 Christ:G5547
(NKJV) in flaming fire taking vengeance on those who do not know God, and on those who do not obey the gospel of our Lord Jesus Christ.
(YLT) in flaming fire, giving vengeance to those not knowing God, and to those not obeying the good news of our Lord Jesus Christ;
2Pe 3:6 Whereby the world that then was, being overflowed with water, perished:
2Pe 3:7
But the heavens and the earth, which are now, by the same word are kept in store, reserved unto fire against the day of judgment and perdition of ungodly men.
2Pe 3:8 But, beloved, be not ignorant of this one thing, that one day is with the Lord as a thousand years, and a thousand years as one day.
2Pe 3:9 The Lord is not slack concerning his promise, as some men count slackness; but is longsuffering to us-ward, not willing that any should perish, but that all should come to repentance.
2Pe 3:10
But the day of the Lord will come as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also and the works that are therein shall be burned up.
2Pe 3:11 Seeing then that all these things shall be dissolved, what manner of persons ought ye to be in all holy conversation and godliness,
2Pe 3:12
Looking for and hasting unto the coming of the day of God, wherein the heavens being on fire shall be dissolved, and the elements shall melt with fervent heat?
2Pe 3:13 Nevertheless we, according to his promise, look for new heavens and a new earth, wherein dwelleth righteousness.
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