The following 2 Thessalonians 2 Chapter was a Warning by Apostle Paul to the 'chosen' elect who would... understand... it. The fact that today many churches show they do NOT understand its simplicity that Apostle Paul used, but instead add outside un-Biblical interpretations to it, reveals those are not 'chosen.'
2 Thess 2:1-15
2 Now we beseech you, brethren, by the coming of our Lord Jesus Christ, and by our gathering together unto him,
Paul's subject is the timing of Christ's future coming and gathering of the Church.
2 That ye be not soon shaken in mind, or be troubled, neither by spirit, nor by word, nor by letter as from us, as that the day of Christ is at hand.
That "day of Christ" phrase is actually about the "day of the Lord" of the Old Testament prophets. In some Greek manuscripts the Greek word kurios for 'lord' is used instead of Greek christos. Per other Scripture, like 1 Thess.5 by Paul, and 2 Pet.3 by Peter, we know that "day of the Lord" is to come "as a thief in the night", and Lord Jesus gave that idea of the thief breaking in as a metaphor for the day of His future coming in Matt.24 and Mark 13. In Revelation 16:15, Lord Jesus said He comes "as a thief".
3 Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition;
4 Who opposeth and exalteth himself above all that is called God, or that is worshipped; so that he as God sitteth in the temple of God, shewing himself that he is God.
That "day of the Lord", i.e., the day of Christ's future coming to gather the Church, shall not come unless those 2 events happen first:
1. "a falling away",
AND...
2. "that man of sin be revealed"
Verse 4 above gives HOW that "man of sin" will be revealed, and that "falling away" is TIED to those acts by that "man of sin", who by the way, has NOT appeared in the world yet today.
Many churches today do not know what that "man of sin" is coming to do, nor what will cause that great "falling away", which is about many believers becoming apostates away from Christ Jesus.
Paul says that "man of sin" will exalt himself, sitting in the "temple of God", showing himself that he is God, and over ALL that is CALLED God, or that is worshiped. Deceived Churches that do not understand that have tried... to claim that "temple of God" there is about the Christian Church when it is definitely NOT! Those doing that are mostly those who support the false idea that a pope is that "man of sin", when it is not.
In Matthew 24:23-26, (which is actually in the singular tense by the way), Jesus warned of this coming "man of sin" where the deceived will say, "Lo, here is Christ, or there;" and Jesus said believe it not. Jesus warned that if they say, "Behold, He is in the desert", or "behold, He is in the secret chambers", believe it not. The phrase in Matt.24:24 "false Christs" is the Greek word pseudochristos. Dr. James Strong in his Strong's Exhaustive Concordance defined pseudochristos to mean 'a spurious Messiah', no doubt because of how the 23rd and 26th verses are in the singular tense about a false Christ.
So there's your actual "man of sin" Paul was pointing to that is to come to Jerusalem at the end of this world, and exalt himself in a new built 3rd stone temple. He will proclaim himself as The Messiah, Christ.
5 Remember ye not, that, when I was yet with you, I told you these things?
6 And now ye know what withholdeth that he might be revealed in his time.
Daniel 10, which reveals a shadow of heavenly events that parallel events happening on earth, also reveals who it is that does that withholding. It is Archangel Michael in Heaven. This is why it will also be Archangel Michael and his angels that will boot Satan and his angels out of Heaven down to our earth at the END, as written about the war in Heaven per Revelation 12:7-17. That war has not happened yet today, but it is soon coming. And by this, Apostle Paul was actually revealing the identity of that "man of sin" will be the devil, Satan himself coming to earth, in person, not hidden, but in plain sight. And the whole world will 'wonder' at him, except Christ's elect will not.
7 For the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way.
Once again, that is Archangel Michael who is withholding Satan in Heaven right now, until it is time for Satan to be booted out of Heaven down to this earth in our earthly dimension. Per Rev.12:7-14, when that happens, he will no longer be able to accuse us in Heaven before God's Throne.
8 And then shall that Wicked be revealed, whom the Lord shall consume with the spirit of His mouth, and shall destroy with the brightness of His coming:
9 Even him, whose coming is after the working of Satan with all power and signs and lying wonders,
Paul reveals that it will be Christ's future coming that will destroy that "man of sin". Satan will play that false-Messiah ROLE that will be destroyed by Christ at His coming. This reveals that false-Messiah comes first to PLAY Jesus Christ to cause the "falling away".
In Matt.24:24, Jesus showed that coming false-Messiah will work great signs and wonders that IF IT WERE POSSIBLE, would deceive even His very elect. Those are the same signs and lying wonders Paul is mentioning here in 2 Thess.2:9, and thus Paul links this "man of sin" with that same spurious-Messiah that Jesus warned of in Matt.24.
10 And with all deceivableness of unrighteousness in them that perish; because they received not the love of the truth, that they might be saved.
11 And for this cause God shall send them strong delusion, that they should believe a lie:
That "strong delusion" of course is this event by that coming false-Messiah-man-of-sin that will show up in Jerusalem at the END of this world, and exalt himself in a new stone temple there, and proclaim himself as God with miracles that will fool the whole world into believing it, except Christ's elect won't believe it (Rev.13:4-8).
And that means many brethren... in the Church will be subject to that "strong delusion", because they are NOT being shown what Paul was actually warning about here, and instead are fed lies that practically go against it; (like the falling away has already begun when it has not because that false-Messiah is not here yet. And their lies that a pope is that false-Messiah, or that the Antichrist is already born in the flesh and is in the world today, when that idea is false too, for Satan will never be born in the flesh to show up on earth with the image of man. They just do not know, so they have to make up stuff, showing they are already deceived in not knowing who that "man of sin" is going to be.)
12 That they all might be damned who believed not the truth, but had pleasure in unrighteousness.
Took me a while to understand how those two 11 & 12 verses could apply to deceived brethren believers on Jesus Christ that will fall away when that false-Messiah comes. God's Word actually does... warn us about Satan coming to earth as that "man of sin". I've already covered quite a few of the pointers right here in this post. Christ's Revelation covers several more showing it will be Satan himself, like in Rev.13 & 17.
13 But we are bound to give thanks alway to God for you, brethren beloved of the Lord, because God hath from the beginning chosen you to salvation through sanctification of the Spirit and belief of the truth:
14 Whereunto He called you by our gospel, to the obtaining of the glory of our Lord Jesus Christ.
15 Therefore, brethren, stand fast, and hold the traditions which ye have been taught, whether by word, or our epistle.
KJV
Do you see the CONDITION Paul gave in that 15th verse? Only IF... we do that, are we in that "chosen" category. And the mere fact that Paul gave that condition in verse 15 reveals that SOME brethren CAN, and WILL fall away at the END of the world by this future coming event of the false-Messiah.