These are some thoughts to consider on 2 Thessalonians c. 2.
The letter was written around 50 - 51 AD during Paul's second missionary trip after having established the church at Thessalonica. (Source: here)
2 Thess. 2:2,
" 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." (KJV)
Paul was reassuring the saints at Thessalonica that the day of Christ was still "at hand". In other words, he was telling them it had not happened yet.
This means that the saints were worried / troubled in mind at the possibility that they may have missed Christ's coming.
If we allow ourselves to realize the full implications of their worry and concern, that as they felt the need to ask Paul if Christ had already returned, then we should understand that they knew the coming of the Lord was not going to be a visible event that would be seen by the entire world!
This directly opposes the modern concept of a coming of the Lord that will be witnessed all over the world by all people of the world.
The words "at hand" meant soon / near to them. It was still out in front of them, yet future for them when Paul wrote the letter to them. He was assuring them that they had not missed it!
2 Thess. 2:3,
"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;" (KJV)
Next Paul told them what to watch for... the revealing of the man of sin. That the man of sin / son of perdition would be revealed to them means that the man of sin was living at the time Paul wrote the letter. The Thessalonians were to watch for that revealing.
This is confirmed in verses 6-7,
"6 And now ye know what withholdeth that he might be revealed in his time. 7 For the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way." (KJV)
The people of Thessalonia knew who was withholding that man of sin, so the man of sin existed in their day, their generation. Paul told them that the mystery of his iniquity was already working when he wrote the letter. And the one "who NOW letteth" is present tense action.. at the time the letter was written in the first century A.D.
Going back to 2 Thess. 2:4,
"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." (KJV)
When Paul wrote this letter, those at Thessalonica, and elsewhere in Palestine, were not thinking of some future "third" temple 2,000 or more years into the future. The reference was to the then standing temple in Jerusalem of the first century A.D.
This verse proves that the temple was still standing when Paul wrote the letter to the church at Thessalonica. The man of sin was someone of their day and time who had not yet fully exposed himself for the evil one that would exalt himself above God.
The saints at Thessalonica knew Christ would return, appearing a second time to them (Heb. 9:28). They were expecting His return in their lifetime just as Christ had promised them (Matt. c. 24, Luke c. 21, etc.). They were seeking to better know when to expect it. Why would they even question Paul in this matter if they did not believe it would happen in their lifetimes?
2 Thess. 2:5,
" Remember ye not, that, when I was yet with you, I told you these things?" (KJV)
That Paul said "remember" means that he had already told them before of this man of sin, and this was a re-telling to soothe their worries.
Verse 7 told them that the mystery of lawlessness was already working as Paul wrote the letter. That is contemporary historical setting of the first century A.D.
So, we must look for the man of sin who caused the people of that day and time to believe a lie with such a strong delusion that many would die because of it (2 Thess. 2:11-12).
Excerpts from "What About Paul's 'Man of Sin'?" by John Noe,
"The best answer-we believe-is that it was both an office (the “what”) and a person (the “one who” or “he”). More specifically, it was the institution of the Jewish priesthood led by Ananus, the high priest. The priesthood opposed the Jewish, Zealot-led rebellion. And Ananus wanted peace with Rome. As long as he and the priesthood stood in the way, the lawlessness of the Jewish Zealots was held back, the “work of Satan” couldn’t reach its full realization, and the “man of sin” couldn’t appear on the scene and cause the final destruction. In A.D. 68, however, Jewish Zealots, with the assistance of the Idumaeans, murdered Ananus and over 12,000 other priests and left their bodies unburied-a violation of the Jewish Law Thus, the priesthood was “taken out of the way” As Josephus wrote in his history of the fall of the city:
'I should not mistake if I said that the death of Ananus was the beginning of the destruction of the city; and that from this very day may be dated the overthrow of her walls, and the ruin of her affairs, whereon they saw their high-priest, and the procurer of their preservation; slain in the midst of their city;. for he was thoroughly sensible that the Romans were not to be conquered. He also foresaw that of necessity a war would follow, and that unless the Jews made up matters with them very dexterously, they would be destroyed: to say all in a word, if Ananus had survived that would have certainly compounded matters… and I cannot but think that it was because God had doomed this city to destruction, as a polluted city, and was resolved to purge his sanctuary by fire, that he cut off these great defenders and wellwishers.'
Verses 8-10. a Ad then the lawless one will be revealed whom the Lord Jesus will overthrow with the breath of his mouth and destroy by the splendor of his coming [parousia]. The coming [parousia] of the lawless one will be in accordance with the work of Satan displayed in all kinds of counterfeit miracles, signs and wonders, and in every sort of evil that deceives those who are perishing. They perish because they refused to love the truth and so be saved!” All this happened in the very Temple that was standing until A.D. 70. As the war between the Jews and Rome developed, a strong leader of the Jewish Zealots emerged who would fulfill Paul’s prophecy. He would soon become the key man in inciting the Jews against Rome, in bringing abominations into the Temple area, and in causing the final destruction of Jerusalem and the Temple. After Ananus’ murder and the removal of the priesthood. Josephus records that a man named John, the son of Levi, fled to Jerusalem from the Roman conquered area of Gischala in Galilee and became the treacherous leader of the Jewish Zealots in control of the Temple area. Also Josephus wrote, “Now this was the work of God, who therefore preserved this John, that he might bring on the destruction of Jerusalem.”
Josephus also records that before this John of Gischala, the son of Levi, was established as the Zealot leader in control of the Temple area (there were three Zealot factions), the power of Satan was already doing his deceitful and treacherous work. This John physically entered the Temple, presented himself to the Zealots as a God-sent ambassador; and persuaded them to defy the laws of Rome and go to war to gain independence. He also instigated the calling in the Idumaeans to keep the Jewish sympathizers from submitting to Rome. He ordered the death of Ananus and the removal of the priesthood. After these atrocities, he became the official leader of the Zealot group m control of theTemple area-john held the temple” and began disregarded the laws of Rome, God, and man, and promising deliverance from the Romans. Then he broke off from the Zealots and began “setting up a monarchial power.” He “set on fire these houses that were full of corn, and of all other provision which would have been sufficient for a siege of many years” He deceived the Jews about the power of the Roman armies In possession of the Temple and the adjoining parts, he cut the throats of anyone suspected of going over to the Romans.13 He performed many sacrileges, such as melting down the sacred utensils used in Temple service, and defiled the Temple."
"After the coming of the Lord and the destruction of Jerusalem and the Temple in A.D. 70, John of Gischala was “condemned to perpetual imprisonment” by the Roman authorities. Thus was fulfilled Paul’s prophetic and symbolic language that this man would be destroyed by “the spirit of his Jesus mouth and brightness of his [parousia] coming” (see Isa. 11:4; 30:27-33; Hos. 6:5; also Da. 7:8, 19-28)."
"In dramatic paralleled fashion, Scripture gives this “man of sin” John of Gischala, the son of Levi- the name of” the one doomed to destruction” or “the son of perdition,” the same name given to another infamous betrayer, Judas Iscariot (compare Jn. 17:12 with 2Th. 2:3 KJV). Both appeared in the same “last days” time frame of the Old Covenant age. Judas betrayed Jesus. John of Gischala betrayed the Jews, fulfilling Paul’s “man of sin” prophecy to a tee."
The full article can be ready here.
All bold emphasis is mine.