In Paul’s second letter to the Thessalonians, he spends time correcting the misunderstanding that the Lord had already returned. So, in the second chapter verses 3 and 4 (NASB) he says, “let no one in any way deceive you, for it will not come unless the apostasy comes first, and the man of lawlessness is revealed, the son of destruction, who opposes and exalts himself above every so-called god or object of worship, so that he takes his seat in the temple of God, displaying himself as being God.” 

Verse 6 then speaks of something that restrains or holds back this lawless one, until it is taken out of the way (verse 7)  “And you know what restrains him now, so that in his time he will be revealed. For the mystery of lawlessness is already at work; only he who now restrains will do so until he is taken out of the way.”

When we look at history, we see the Roman Empire at that time had conquered the world and had not only ruled over politics and laws but that the emperor was also ruler over religious affairs. Roman emperors were worshipped and given the title “Pontifex Maximus” meaning the Chief Religious Leader.  When Constantine became emperor in the fourth century, he established Christianity as the religion of the land. Up until this time the Roman Empire persecuted Christians.

As the Roman Empire began its decline, the Antichrist grew in its influence.  In the fifth century, the Bishop of Rome, Leo the Great,  was given the title Pontifex Maximus. In the sixth century, the Bishop of Rome was given general oversight of all the Christian churches as well as political jurisdiction in Rome.

So gradually, the Roman Empire (called “he who now restrains”) was taken out of the way so that the Church of Rome (Papacy, the Antichrist) could take control. The Church of Rome was no longer restrained. This mystery of lawlessness which had already been at work in the apostles’ day (v.7) could now exercise control.  This “mystery”, not obeying God’s laws (or lawless), became the persecutor of the true church. It earned the title given it in Revelation: “Mystery, Babylon the Great, the mother of harlots and of the abominations of the earth”.  Revelation 17:5

Additional Resource: (PDF) “Can We Identify Antichrist?”