The question was asked “Who is the Man of Lawlessness”? To answer the question I first turn to the apostle John:
1 John 2:18-19 Children, it is the last hour, and as you have heard that antichrist is coming, so now many antichrists have come. Therefore we know that it is the last hour. They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, that it might become plain that they all are not of us.
1) It is the last hour and 2) many antichrists have come. Then John writes:
1 John 4:1-3 Beloved, do not believe every spirit, but test the spirits to see whether they are from God, for many false prophets have gone out into the world. By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God, and every spirit that does not confess Jesus is not from God. This is the spirit of the antichrist, which you heard was coming and now is in the world already.
Observations: 1) the one who does not confess Jesus is not from God 2) This is the antichrist and 3) this antichrist was coming and is existing at the time of the 1st century. There is suggestion that the antichrist is one who works to substitute Christ and/or becomes a false Christ. He is a false messiah and a false confession.
Paul writes concerning the Man of Lawlessness:
2Thessalonians 2:3–11 Let no one deceive you in any way. For that day will not come, unless the rebellion comes first, and the man of lawlessness is revealed, the son of destruction, who opposes and exalts himself against every so-called god or object of worship, so that he takes his seat in the temple of God, proclaiming himself to be God. Do you not remember that when I was still with you I told you these things? And you know what is restraining him now so that he may be revealed in his time. For the mystery of lawlessness is already at work. Only he who now restrains it will do so until he is out of the way. And then the lawless one will be revealed, whom the Lord Jesus will kill with the breath of his mouth and bring to nothing by the appearance of his coming. The coming of the lawless one is by the activity of Satan with all power and false signs and wonders, and with all wicked deception for those who are perishing, because they refused to love the truth and so be saved. Therefore God sends them a strong delusion, so that they may believe what is false,
The context in which Paul is writing about is in response to trouble that was occurring in the Thessalonican community. False rumors had spread about Jesus having already came. Paul was trying to remind them that certain things had to happen before the return of Jesus. Paul is telling them that before the return of Christ there had to be a revealing of the this Man of Lawlessness.
Many commentators believe Paul is describing the same thing John was describing in his letter of 1 John (antichrist). Because he is 1) opposes Christ. He is against Christ. 2) He exalts himself above all that is God and demands that he is worshipped. This Man of Lawlessness is antichrist.
Note that he takes his seat in the temple of God. This is either occurring while the Herodian temple was still standing. Or it was referring to a future event that would require the building of the temple in the land of Israel. Obviously the Dispensationalist wants to see this as a future event that will happen in the future temple to be built in Israel.
John spoke about the antichrist being at work already. In verse 7 of 2 Thessalonians the Man of Lawlessness is being presently restrained and then he will be revealed. The coming of the Man of Lawlessness is antichrist who is one and the same thing.