Theologians are divided about the meaning of these mysterious words of St. Paul. “What” is it that restrains? And who is “he who now restrains?“ The Early Church Fathers often held that the restrainer was the Roman Empire, based on Daniel 7:24:
Out of this kingdom ten kings shall arise, and another shall arise after them; he shall be different from the former ones, and shall put down three kings. (Dan 7:24)
Now this restraining power [is] generally admitted to be the Roman empire… I do not grant that the Roman empire is gone. Far from it: the Roman empire remains even to this day. —Blessed Cardinal John Henry Newman (1801-1890), Advent Sermons on Antichrist, Sermon I
And yet, St. Paul also refers to “he who restrains,” as in a person or possibly angelic entity. From the Navarre biblical commentary:
*Although it is not altogether clear what St. Paul means here (commentators ancient and modern have offered all kinds of interpretations), the general thrust of his remarks seems clear enough: he is exhorting people to persevere in doing good, because that is the best way to avoid doing evil (evil being the “mystery of lawlessness”). However, it is difficult to say precisely what this mystery of lawlessness consists in or who is restraining it.
Some commentators think that the mystery of lawlessness is the activity of the man of lawlessness, which is being restrained by the rigid laws enforced by the Roman Empire. Others suggest that St. Michael is the one who is holding lawlessness back (cf. Rev 12:1; Rev 12:7-9; 20:1-3, 7)… which show him combating Satan, restraining him or letting him free… others think that the curb on the man of lawlessness is the active presence of Christians in the world, who through word and example bring Christ’s teaching and grace to many. If Christians let their zeal grow cold (this interpretation says), then the curb on evil will cease to apply and the rebellion will ensue.* —The Navarre Bible commentary on 2 Thess 2:6-7, Thessalonians and Pastoral Epistles, p. 69-70
. . .]
Today, the Roman Empire is believed to subsist in some form through the European Union, which embraced the Treaty of Rome in forming its economic union. America, I might add, finds its roots in the European people, and through a nearly constant history of war, has built an empire of sorts throughout the Middle East and beyond. Others believe the Roman Empire has yet to rise in its final form before it falls for good.