KJV 2Th 2:1 Now we beseech you, brethren, by the coming of our Lord Jesus Christ, and by our gathering together unto him, 2Th 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. 2Th 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; 2Th 2:4 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. 2Th 2:5 Remember ye not, that, when I was yet with you, I told you these things? 2Th 2:6 And now ye know what withholdeth that he might be revealed in his time. 2Th 2:7 For the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way. 2Th 2:8 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: 2Th 2:9 Even him, whose coming is after the working of Satan with all power and signs and lying wonders, 2Th 2:10 And with all deceivableness of unrighteousness in them that perish; because they received not the love of the truth, that they might be saved.
The Preterist Archive gives this answer, among others.
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.
They also say:
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.
Wikipedia notes that John 17:12 also uses the term "son of perdition," referring to Judas. It also gives this information.
Some scholars and theologians down through history, including Hippolytus, Luther, Wesley, Manton, Schaff, et al, say that first "Son of Perdition" reference is to Antiochus IV Epiphanes, the man who attacked the Second Temple in Jerusalem and defiled it by sacrificing a pig on the altar, erecting a statue of Zeus as himself in the temple, raiding the Temple treasury and minting coins saying "Theos Epiphanes" (God manifest), etc. Even those theologians who advocate an interpretation of Daniel that includes the Roman Empire in their analysis recognize Antiochus as a prototype.
Preterism is a fairly wide position to identify only one view. For example, some believe all prophecy is fulfilled, while others believe only parts are fulfilled historically and some remain.
To the OP specifically and 2 Thes. 2:3, the primary interpretation rests on understanding on to what the temple of God refers.
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 Thes. 2:4
For Paul and the others, the temple of God was not the physical Jerusalem temple still standing until CE 70. They all knew that physical temple was empty and void. God never indwelt that temple. It was not the temple of God.
Instead, the temple of God was the believer.
1 Cor. 3:16 Know ye not that ye are the temple of God, and that the Spirit of God dwelleth in you?
2 Cor. 6:16 And what agreement hath the temple of God with idols? for ye are the temple of the living God; as God hath said, I will dwell in them, and walk in them; and I will be their God, and they shall be my people.
Rev. 3:12 Him that overcometh will I make a pillar in the temple of my God, and he shall go no more out: and I will write upon him the name of my God, and the name of the city of my God, which is new Jerusalem, which cometh down out of heaven from my God: and I will write upon him my new name.
With that in mind, the only possible fulfillment of 2 Thes. 2:3 would be from a Christian who exalts himself into the position of God within the temple (body) of God.
Chrysostom, Tertullian, and others understood this. They wanted the Roman Empire to remain in authority. It "withheld". They understood that once the Roman Empire fell, the way to fulfill 2 Thes. 2:3 was opened. They didn't see CE 70 as a fulfillment of the Pauline man of sin/perdition. Judas was a "believer".
One may naturally enquire, what is that which withholdeth, and after that would know, why Paul expresses it so obscurely. What then is it that withholdeth, that is, hindereth him from being revealed? Some indeed say, the grace of the Spirit, but others the Roman empire, to whom I most of all accede. -Chrysostom-
So, while some preterists not understanding the "temple of God" believe all was fulfilled at CE 70, others believe it was fulfilled centuries later and know who it is.
I don't think we can identify for certain who this son of perdition was. However, we can know when he was, and most probably of whom he was - a Zealot.
Scholars generally place both letters to the assembly at Thessalonica between 50 - 51 AD.(1) Different speculations have been offered about who the man of sin was, also called the son of perdition (destruction) in the same verse at 2 Thess. 2:3, because they forget the time factor of the day of Christ in vs 2. The day of Christ was "at hand" when the letter to the Thessalonians was written. Paul was telling them that Christ was going to come in the not very distant future, or soon.
The destruction of the temple in A.D. 70 would then have been about 19 - 20 years after Paul's letter to the Thessalonians. This window of time allows for several possibilities centering around those who could have occupied, or sat in the temple during those years.
That the man of sin would be sitting in the temple, exalting himself above God means that the temple was still standing, or he could not be sitting in it. This rules out anyone after the destruction of that temple after AD 70 (CE 70).
Adam Clarke links these verses back to Isa. 1:4, benim mashchithim, children of perdition and destruction. (2)
"Ah, sinning nation, a people heavy [with] iniquity, A seed of evil doers, sons -- corrupters! They have forsaken Jehovah, They have despised the Holy One of Israel, They have gone away backward." (YLT)
A case of double fulfillment would have the sinning ones, the corrupters who turned away from God in Isaiah's day as those who repeated the same action that rejected and turned away from His Son in the first century A.D. - the Jews. So, the man of sin, son of perdition, the lawless one was one of the Jews.
The Roman emperors never ruled in that temple, and is thus another limiting factor. The one who restrains mentioned in vs.6-7 was someone Paul had already told them of, but for reasons of their safety did not name in the scriptures. It is sufficient for us to know that they knew who that person was.
That they knew who it was means that person restraining the son of perdition already existed and was already in some position of authority, otherwise he could not restrain anything or anyone.
Possibilities for that office of authority could have been the Roman governor appointed over that region - Herod Agrippa II - who tried to mediate between the Zealots and Rome to keep peace.(3) Some have claimed it was Emperor Claudius who had expelled the Jews from Rome in 50-51 AD (Acts 18:2) for causing riots over "Chrestus." (4)
The wicked son of perdition had to be someone who could have sat in that temple, and therefore had to be from within Jerusalem before the temple was destroyed. That person could only have been either one of the high priests, or one who falsely claimed to be sitting in place of the high priest. Therefore, a Jew.
Paul was reminding the Thessalonians that he had told them earlier (vs. 5), so that their questions about when Christ was coming was not the first time this subject had been raised. They knew the prophesy of the destruction of the temple (Matt. 24), and Paul was telling them again that certain things had to happen before Christ came to destroy it.
One of those things was the falling away of the new Christians back into the old law, which happened because of the constant push from the Sadducees and Pharisees. This pressure to return, or fall away would not be so prevalent, or even exist after the destruction of the temple worship system, after AD 70. Therefore again the time frame of the revealing of that Wicked one would be before Christ came to destroy that temple.
He was described as a liar and deceiver, after the workings of Satan in vs. 9 and 10. "Them that perish"... those that received not the love of the truth, and rejected the Messiah, would not be saved.... not necessarily just the Jews, but most probably centered around the unbelieving Jews who influenced any that also denied Jesus as their savior.
All of these timing factors place this man of sin, the one that exalted himself as God and opposing God to have been in Jerusalem before the destruction of that temple in AD 70. The first answer provided by Allan above offers John of Gishcala, a Zealot who was one of the leaders that fomented the rebellion against Rome in 66-67 AD. His cohort was Simon Bar-Giora.
John of Gishcala was a deceiver, one who feigned friendship with Ananas, a former high priest who had tried to get the Jews to throw off the Zealots who had captured the temple.(5) John of Gishcala then informed his cohorts inside the temple of Ananas' plans, and called upon the Idumeans (Edomites) to come to their aid in their rebellion against Roman rule.
John had been an enemy of Josephus during the time that Josephus was the Jewish general over Galilee in the first Roman-Jewish war of AD 66 -70.(6) He could very well have sat and ruled in the temple during the time before its destruction. But, there is one problem with his selection for this role of the son of perdition.
2 Thess. 2:8 says that the Lord would consume and destroy the Wicked lawless one at His coming. This would indicate that the son of perdition and destruction would in turn be destroyed, killed when Christ came to destroy the temple. That would rule out both John of Gishcala and Simon Bar-Giora, as both survived the war and were marched back to Rome under Titus' victory procession. John of Gishcala lived the rest of his live in prison.(7)
Except for what Josephus recorded, much of the secular history of this time period is lost to us, and the Bible only recorded those events that are necessary for our reconciliation to God under His plan of salvation in Christ.
Therefore, the answer to this question can only be speculation from within a certain group of people that had opportunity to act against God during a certain time period before Christ's return to destroy that temple.
All that we can say with some certainty is that the son of perdition, the lawless one was most probably that Zealot that caused the most destruction, that exalted himself as God during the occupation of the temple before its destruction.
A fair overview of the Zealot revolt of the first Roman-Jewish war can be found at That The World May Know here
Another worthwhile summary reading for the Roman-Jewish revolt and war can be found at the JewishVirtualLibrary here