From previous verses we learn that Paul is talking about the Antichrist being revealed. Apparently he had talked to the church earlier about some power that was preventing the full revelation of that man of sin. In verse 7 also, Paul refers to a certain entity who would cease withholding by being "taken out of the way."
Unfortunately, these meagre clues do not identify the information that had been previously discussed with the Thessalonians. Paul's guarded language seems to indicate that it might be a politically explosive issue, and he avoided calling the restraining power by name.
Many believe that the epistle is referring to the authoritarian influence of the pagan Roman empire, which did indeed hinder the growth and development of the papal religious system. With its fall in A.D. 476, the way was cleared for a rapid world domination of the papal church, which continued until 1798.