The word for "mystery" is the Greek
musterion and means "hidden thing." Sin is not hidden, but is obvious. Satan's work in the world is not hidden, but obvious. Therefore, this must be talking about something else, but the rest of the paragraph in 2 Thessalonians 2 makes it clear what that is.
2 Thessalonians 2 NASB
2 that you not be quickly shaken from your composure or be disturbed either by a spirit or a message or a letter as if from us, to the effect that the day of the Lord has come.
3 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,
4 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.
5 Do you not remember that while I was still with you, I was telling you these things? 6 And you know what restrains him now, so that in his time he will be revealed.
7 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.
8 Then that lawless one will be revealed whom the Lord will slay with the breath of His mouth and bring to an end by the appearance of His coming;
9 that is, the one whose coming is in accord with the activity of Satan, with all power and signs and false wonders,
10 and with all the deception of wickedness for those who perish, because they did not receive the love of the truth so as to be saved.
11 For this reason God will send upon them a deluding influence so that they will believe what is false,
12 in order that they all may be judged who did not believe the truth, but took pleasure in wickedness.
Note that when the "man of lawlessness" in v. 3 "will be revealed" is the sign that the apostasy (v. 2) begins. It is no accident that the word "lawlessness,"
anomia in the Greek, is used in both this instance and in your v. 7, the focus of your inquiry. The man of lawlessness and the mystery of lawlessness are linked. The mystery is his identity, for he is the antichrist. It will not become a solved mystery until that time when the rapture (v. 6) occurs and, sometime afterwards, the antichirst is revealed and the Tribulation will begin.