It is always correct to use the first audience perspective when reading the Scriptures. We know when reading the accounts from the Old Testament that we are reading historical events that happened long ago. But, somehow when we begin reading the accounts from the New Testament many people start applying the words written almost 2,000 years ago to our current generation.
Paul was comforting those of the assembly at Thessalonica who were believers of Christ in the 1st century AD. The two letters Paul wrote to the Thessalonians were written approx. 50 - 51 AD. (1) We know from 2 Thess. 2:4 that the temple was still standing in Jerusalem at the time the letter was written. The Herodian temple at Jerusalem was destroyed by the Romans in 70 AD. So, the letter predates the destruction of Jerusalem.
The assembly was concerned that the "day of the Lord" had already happened. This idea could only have been promoted in Thessalonica by a false teacher, or false prophet, and Paul was assuring them that they had not missed it. Christ had not yet returned when Paul wrote this letter about 51 AD. And, he was letting them know that certain events were just about to happen, specifically the man of sin that would sit in that Jerusalem temple and claim to be God.
"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." (2 Thess. 2:3-4, KJV))
As there is no Herodian temple in existence today in Jerusalem, then Paul's assurance to the Thessalonians would have no meaning to anyone in Thessalonica today. Further, Paul assured them that the "man of sin" was alive and working when he wrote that letter in the first century AD.
"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. 7 For the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way." (2 Thess. 2:5-7 KJV)
"Now letteth", present tense when the letter was written. Compare to the YLT:
"7 for the secret of the lawlessness doth already work, only he who is keeping down now [will hinder] -- till he may be out of the way,"
There are many who speculate about the man who was restraining the one who was working lawlessness, and there are some candidates proposed. But, the key here was that this man of iniquity would sit in the temple in Jerusalem and claim to be God. That could only have happened while that temple still stood, sometime before its destruction in AD 70. And, it would have to have been one of the men who claimed to be a High Priest in the Jerusalem temple.
We do not have a written record of the spoken words of the men ruling within Jerusalem during its last days of the siege during the Roman-Jewish wars of 67-70 AD, so we do not know if it was the appointed high priest Phinehas ben Samuel of Havta, who was chosen by the Zealots through the casting of lots, who made that false claim to be God, or if it was another. (2) (3)
Paul's point was that the events were even on-going when he wrote to Thessonica in 50-51 AD. They were happening during their lifetime, in their generation. It is not applicable to us in our generation. The lawless one was going to be destroyed on the day of judgment that God appointed for the destruction of that temple which Christ promised would happen in their generation (Matt. 24), and which we know was destroyed in AD 70.
Christ's second appearance (Heb. 9:28) was promised to the same generation that saw His first appearance / manifestation during the 1st century AD, and could not have happened to any other generation after the the 1st century AD, as no other generation saw His first manifestation. It is mathematically impossible to have a second occurrence when there is not a first occurrence.
The Thessalonians would not have been concerned that they had missed the day of judgment of Christ's second appearance if they had not been already told that He was returning in their lifetime. Paul was assuring them they would live to see Christ's return.
"8 and then shall be revealed the Lawless One, whom the Lord shall consume with the spirit of his mouth, and shall destroy with the manifestation of his presence," (2 Thess. 2:8, YLT)
That day of judgment was promised to THEM, and was set for that generation (Matt. 24:34; Mark 13:30; Luke 21: 22-24; all of Revelation). God sent the unbelieving Jews a delusion (vs. 10-12) which played out during the siege by the continual false prophets who lied to the people in that city making them believe that God would protect them. The people in Jerusalem became insane in their continued resistance and rebellion of that war. (4) (5)
But, to those who believed Christ, and kept the faith was promised salvation and deliverance (2 Thess. 2:13-15). And, that deliverance came with the death of their persecutors. (6) When the temple was destroyed, the Sanhedrin was no longer able to exert their influence on Rome and the severe tribulation and persecution the believers had been subjected to under Nero was removed.
Paul's letter to the Thessalonians was for THEIR comfort and THEIR assurance, and we can be assured that every word of it was true.
Dating The New Testament - here
The last high priest at the temple - Chabad
High Priests of the 2nd Temple - here
Baseless hatred - here
Josephus, Wars Bk VI - here
Which Salvation Arrived in AD 70 - ShreddingTheVeil