The main Message by Apostle Paul in 2 Thessalonians 2 was given by Paul as a correction... to the Thessalonian Church because some false ones among them were spreading lies against the order of Christ's future coming and gathering of His Church.
2 Thess 2:1-5
2 Now we beseech you, brethren, by the coming of our Lord Jesus Christ, and by our gathering together unto Him,
Paul begins by pointing to the subject of Christ's coming and gathering of the Church to Him.
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 "nor by letter as from us" points to false crept in unawares among the Thessalonians that were trying to confuse the timing of Christ's coming and gathering of His Church. We still have those trying to do that today in many Churches, particularly those on the devil's false Pre-tribulational Rapture theory.
Paul then points directly to Christ's coming on "the day of Christ", which is actually "the day of the Lord" per the original Greek. That "day of the Lord" per Scripture is for the very LAST DAY of this present world. Thus Paul gave the correct timing for Christ's future coming, AFTER the tribulation like Jesus Himself said in Matthew 24:29-31 and Mark 13:24-27.
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;
Paul then continues about Christ's coming on "that day" (the "day of the Lord"), that it will not happen until first... there comes the "falling away" (apostasia), and the "man of sin" (Antichrist) is revealed...
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.
Paul then continues that idea further with defining HOW... that "man of sin" will be 'revealed'. That "temple of God" is about another Jewish stone temple to be built in Jerusalem for the end when the Antichrist comes at the end of this world. Jesus gave this same kind of warning about this coming false one to Jerusalem that the deceived will be claiming is The Christ (see Matthew 24:23-26, pointing to a singular false-Christ).
5 Remember ye not, that, when I was yet with you, I told you these things?
KJV
Paul then remarks to them how he had told them about these things previously.
It is about the coming of a false-Messiah to Jerusalem first, and being setup as The Christ, with the whole world being deceived into worshiping the "dragon", except those who's names were written in the book of life from the foundation of the world. (Revelation 13:4-8). That is what that "falling away" will be that Paul pointed to even with the idea of God sending the deceived "strong delusion".