It was already in existence in the first century
of our present age.

It is evidently literally a mystery in the ancient Greek sense. That is to say that it is secret or semi-secret religion whose teaching are kept secret from the public. Sometimes such religions had a public presence, but at other times were completely secret, even in their very existence. There have been many such religions since ancient times, and to quote the dictionaries, they tended to be “relig. and political in nature,” A Greek-English Lexicon of the New Testament and Other Early Christian Literature, Arndt and Gringrich, The University of Chicago Press, 1957, pg 531-532.

The Mystery of Lawlessness is evidently the same thing
as the beast organization in Rev 13:1-3.

Then one of the heads of the beast-organization, becomes “the beast” (Rev 13:3) / “the man of lawlessness (2Thes 2:8). This man of lawlessness, this beast, is worshipped by the entire world, Rev 13:4 and 2Thes 2:4, 9-12.

This is evidently a Satanic mystery religion, and Scripture clearly indicates that it both cooperates with Mystery Babylon the Great at times, and also competes with and opposes Mystery Babylon the Great at times. Paul says in the first century of our age that,

“For the mystery of lawlessness already works. …”
2Thes 2:7a WEB

It was already in existence and has worked for over 2,000 years.

The purpose of the Mystery of Lawlessness is:

“3 … the man of sin is revealed, the son of destruction, 4 he who opposes and exalts himself against all that is called God or that is worshiped; so that he sits as God in the temple of God, setting himself up as God.”
2Thes 2:3b-4 WEB

This is basically the same as is described in Revelation 13. There it says,

“It was given to him to make war with the saints, and to overcome them. Authority over every tribe, people, language, and nation was given to him.”
Rev 13:7 WEB

This is for a relatively short, but terrible times, just before the end of this universe.

The man of lawlessness WAS ready to be revealed
in the first century of our age.

And Paul said to the Thessalonians,

“Now you know what is restraining him, to the end that he may be revealed in his own season.”
2Thes 2:6 WEB

The Thessalonians knew what was restraining him, but we do not, but he has been being restrained for over 2,000 years, and one day he will be revealed, and there have been continual attempts seat such a man, Christian-Age-long. See the section on “Lab Work,” in Prophecy Principles.

And he will be destroyed by the Second Coming of Jesus Christ (2Thes 2:8, Rev 19:20).

Thus the case for the Mystery of Lawlessness is the most straight forward of all.

So these are powers which Christians of every age
have had to contend..

The same is true for that other great age-long enemy of Christianity, Mystery Babylon the Great. It is evidently in existence in the first century of our present age, and so will until near the very end of this present universe, and is destroyed by the beast and ten kings, Rev 17:18.

WEB is the World English Bible, a copyright free revision
of the original ASV American Standard Version 1901