fixed' now in heaven. (Luke 16). Read All.
     This 'mystery of iniquity' are Satan's own, the Kenites (sons of Cain), who prepare the deceived world to accept Satan as Messiah. (Make sure you separate these two objects in verse 7, one being 'the mystery of iniquity' already at work, then note the colon and change of subject back to Satan being loosed out of the pit by the Archangel.) This 'mystery of iniquity' is also known as the 'spirit of antichrist' already at work during the Apostle's time.

1 John 2:18: Then answered the Jews (Pharisees) and said unto Him, “What sign shewest Thou unto us, seeing that thou doest these things?”

1 John 2:19: Jesus answered and said unto them,” Destroy this Temple, and in three days I will raise it up.” (Temple being His flesh body.)

1 John 4:3: And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard, that it should come; and even now already is it in the world.

 It's the offspring of the serpent (Satan), the same group of deceivers that spurned the killing of God's prophets and Jesus Christ Himself on the cross. They are the 'beasts' of Rev.6:8:

 

The 'generation of vipers' of...

Matt.23:33:  Ye serpents, ye generation of vipers, how can ye escape the damnation of hell.

 i.e. the race of Kenites.
      I've seen some deceived Churches teach that...

 I John 2:18: Little children, it is the last time: and as ye have heard that antichrist shall come, even now are there many antichrists; whereby we know it is the last one.

...Means there is no one particular being called 'The Antichrist'. They teach John was saying that since the 'spirit of antichrist' was already in the world, then that was 'The Antichrist'. And since Paul is teaching here in...

 2 Thess: 3:  Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition.

...That 'the son of perdition' must come first, these people say that antichrist has already come to earth, and thus... the 'rapture' can happen at 'any moment'. Not so.

That's a false doctrine to get many to worship the false messiah who comes prior to Jesus Christ, by believing in the false pre-tribulation  fly-away "secret rapture" theory. To accept that false 'any moment' doctrine, they even throw away the seven signs which our Lord and Savior gave us in His Gospel, and in His Book of Revelation (compare Mark 13; Matt.24; Luke 21 with Rev.6).  These workers of iniquity are guided by Satan's evil 'spirit of antichrist', for he is The Antichrist. In contrast to the 'workers of iniquity', our Father works through His People using The Holy Spirit.

II Thess 2:8    And then shall that wicked be revealed, whom the Lord shall consume with the spirit of His mouth, and shall destroy with the brightness of His coming: