15 Grk “the mystery of lawlessness.” In Paul “mystery” often means “revealed truth, something formerly hidden but now made widely known,” but that does not make sense with the verb of this clause (“to be at work, to be active”).
16 Grk “the one who restrains.” This gives a puzzling contrast to the impersonal phrase in v. 6 (“the thing that restrains”). The restraint can be spoken of as a force or as a person. Some have taken this to mean the Roman Empire in particular or human government in general, since these are forces that can also be seen embodied in a person, the emperor or governing head. But apocalyptic texts like Revelation and Daniel portray human government of the end times as under Satanic control, not holding back his influence. Also the power to hold back Satanic forces can only come from God. So others understand this restraint to be some force from God: the preaching of the gospel or the working of the Holy Spirit through God’s people.

In other versions

For the mystery of lawlessness is already at work. Only he who now restrains it will do so until he is out of the way.

For the secret power of lawlessness is already at work; but the one who now holds it back will continue to do so till he is taken out of the way.

For the mystery of lawlessness is already at work, but the one now restraining will do so until he is out of the way,

For this lawlessness is already at work secretly, and it will remain secret until the one who is holding it back steps out of the way.

For the mystery of lawlessness is at work already; only the one who now restrains will do so until he is out of the way,

For the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way.

Public Domain