The "Man of Sin" is the Antichrist who at the very close of this Age will oppose and exalt 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.2:4). In the seventh verse of this same chapter the apostle tells us "For the mystery of iniquity doth already work: only He who now letteth (hindereth) will let (hinder), until He be taken out of the way." Notice that the "Mystery of Iniquity" was said to "work" referring to the action of the leaven which was corrupting the meal even in the apostle's own lifetime. That which has "hindered" the full development of the Mystery of Iniquity" and which now prevents the revelation of the Antichrist is the presence on earth of God the Holy Spirit. But He is to be "taken out of the way" -- removed from the earth when the saints are raptured to heaven. Then will the Devil be allowed "free rein" and the Son of Perdition will be publicly manifested. In the days of Antichrist God will send men strong delusion "that they should believe a lie: that they all might be damned, who believed not the truth, but had pleasure in unrighteousness" (vs.12). The career of the Antichrist will be cut short by the return of our Redeemer to the earth -- "whom the Lord shall consume with the Spirit of His mouth, and shall destroy with the brightness of His coming" (vs.8). The "Mystery of Iniquity" then, will be brought to a conclusion only by the Return of Christ in judgment which is another proof that there can be no Millennium before the Second Coming of Christ.
To sum up the teaching of 2 Thess.1:6-2:12. The testimony of this passage is in perfect accord with the declarations of the Lord Jesus in Matthew 24. Instead of teaching that before Christ returns all men will be converted by the Gospel, it distinctly affirms that the Day of Christ (the Millennium shall not come "except there come a falling away first" (Greek "apostasy"). Instead of teaching that this Age will close by witnessing a universal turning unto the Truth it explicitly states that it will terminate with God giving up multitudes "that they should believe a lie." Instead of teaching that this Dispensation will end with Christ exalted in the hearts of all, it declares that it will close with the manifestation and exaltation of the Antichrist and with the Lord coming back in judgment to destroy the Wicked One and to take vengeance on those that know not God and have scorned the Gospel of His Son.