Even this guy's name sounds like bad news. So who is he? And why is everyone so interested in his wicked ways?
The Man of Sin gets his one and only mention in the Bible in 2 Thessalonians:
Let no one deceive you in any way; for that day will not come unless the rebellion comes first and the lawless one is revealed, the one destined for destruction. He opposes and exalts himself above every so-called god or object of worship, so that he takes his seat in the temple of God, declaring himself to be God[…] And you know what is now restraining him, so that he may be revealed when his time comes. For the mystery of lawlessness is already at work, but only until the one who now restrains it is removed. And then the lawless one will be revealed, whom the Lord Jesus will destroy with the breath of his mouth, annihilating him by the manifestation of his coming. (2 Thessalonians 2:3-4, 6-8)
This guy goes by different names depending on what translation of the Bible you're using. The New Revised Standard Version, which we just quoted, calls him "the lawless one." Catchy. But, the King James translation calls him "that man of sin," and that's the name that got the most traction.
The verse says that the Man of Sin is tied to the second coming of Jesus. Basically, Jesus won't come back until the "rebellion" starts and the Man of Sin is shown to be who he really is—a super sinful guy. "Rebellion" can also be translated "apostasy," which just means there will be a time when Christians start to turn away from their faith in Jesus. Boy, are they gonna feel silly when Jesus descends from Heaven and starts laying down his wrath left and right.
The author also tells us that the Man of Sin sets himself up in the place of God in the temple. That makes him like Antiochus IV Epiphanes, who tried to force the Jews to worship Zeus in the temple in Jerusalem (see 2 Maccabees) or Caligula, who wanted to have statues of himself put up inside the temple. Those are big no-nos, guys.
Things aren't looking so good for the lawless one.
Currently, the reason no one knows he exists (except for the author and the friends he's writing to) is that someone or something is holding him back. The author says that the community knows who this is…but do we? Is it God? Jesus? Or maybe even the Roman Empire itself? Whoever it is, soon that restrainer is going to fall by the wayside and everyone will understand just who this big sinful dude is.
Naturally, once everyone notices him in their midst, Jesus will come back (complete with angels and trumpets) and kill the Man of Sin "with the breath of his mouth." But that's not just heavenly halitosis. That image recalls the Hebrew Bible—"with the breath of his lips he shall kill the wicked" (Isaiah 11:4). (Source, p. 1216-17.)
The short answer to this question is: we've got no clue.
Is this guy a historical figure like the Roman Emperor? Is he some kind of supernatural entity like Satan? Or is he just a random "false prophet" spreading bad info about Jesus like the ones Mark warns against in his gospel (Mark 13:21-22)?
In an effort to figure this guy out, some people have tied him to the Antichrist mentioned in the Epistles of John. But that seems unlikely, since the author of those letters is calling out false prophets in his own community. How about one of the beasts in Revelation 13? Again, those guys are probably symbolic representatives of Rome, and the author of 2 Thessalonians really isn't that political, so we doubt he's calling out the whole Empire.
Whoever he is, the author obviously thought the folks he was writing to would pick him out right away. It's sort of like a secret code he used to avoid naming names. (Does that mean the Man of Sin could also be called He Who Must Not Be Named?) The community would have known who the Man of Sin was (and known he was no good), but two thousand years later, we just don't have any way to crack the code. (Source, p. 1216-17.)