Who is the man of sin in 2 Thessalonians 2?
The man of sin in 2 Thessalonians 2 is the Antichrist, a human figure who will be revealed after a falling away in the church and who exalts himself above God, even sitting in God's temple and declaring himself to be God.
Scripture calls him the man of sin, the son of perdition, because he embodies rebellion against God in its fullest form. Paul teaches that the day of the Lord cannot come until there is first a falling away and this man of sin is revealed, showing that his appearance is a necessary event on the timeline before that day begins. He is described as one who opposes and exalts himself above all that is called God or is worshipped, so that he sits in the temple of God, showing himself that he is God.
His coming is tied to the working of Satan, with all power and signs and lying wonders, and with deceivableness of unrighteousness in those who perish because they did not receive the love of the truth. He will look and sound like an ordinary man, yet he is in fact a manifestation of the devil himself, just as Jesus is the embodiment of God. The mystery of iniquity that produces him is already at work in the world, restrained only until the one who withholds him is taken out of the way, at which point that Wicked one will be revealed.
When he is finally revealed, the Lord will consume him with the spirit of His mouth and destroy him with the brightness of His coming. Until then, the falling away within the church, marked by persecution, offenses, false teaching, and love of the world, is described as the setting in which this man of sin is being prepared to be revealed.
The scriptures
Now we beseech you, brethren, by the coming of our Lord Jesus Christ, and by our gathering together unto him, 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. 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; 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.
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. Remember ye not, that, when I was yet with you, I told you these things? And now ye know what withholdeth that he might be revealed in his time. For the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way. 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: Even him, whose coming is after the working of Satan with all power and signs and lying wonders,
For the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way. 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: Even him, whose coming is after the working of Satan with all power and signs and lying wonders, And with all deceivableness of unrighteousness in them that perish; because they received not the love of the truth, that they might be saved.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.