2 Thessalonians 2:7

For this mysterious lawlessness is already at work [or, "this lawlessness is already secretly at work"]; but the one who is holding it back [will continue to do so] until he is removed.

Revelation 17:5

And on her forehead was written a mysterious [name], "The great Babylon, the mother of the prostitutes and the disgusting things of the earth."

Revelation 17:7

Then the angel [See verse 1] said to me, "Why are you so amazed? I will explain to you the secret [meaning] of the woman and the beast with seven heads and ten horns that she is riding on.

1 John 2:18

Dear children, it is [now] the last hour [i.e., the final period of God's dealing with people on earth], and since you have heard that an antichrist [i.e., a person who is against Christ] is coming, even now [you should be aware that] many antichrists have [already] come. This is [one way] we know that this is the last hour.

1 John 4:3

But every spirit who does not proclaim [this truth about] Jesus is not [speaking] from God. And this [person] represents the spirit of the antichrist [See 2:18-22; II John 7], whom you have heard would come. Well, now he is already here!

Acts 20:29

[Because] I know that after I am gone [from here] there will be vicious wolves [i.e., ravaging false teachers] who will come in among you, and they will not spare the flock [i.e., the congregation].

Colossians 2:18-23

Do not allow anyone, who delights in acting humble [i.e., living an ascetic life] and who worships angels, to rob you of your reward [in heaven]. [Note: This "angel worship" could mean holding angels in high regard because of their role as God's messengers, or the claim that God can be approached only through angelic intermediaries]. He places great importance on what he [claims to] see [i.e., visions], being inflated with pride by his fleshly mind for no [good] reason.

1 Timothy 3:16

And without question the revealed secret of godly living is great. [It is this]: He [i.e., Christ, or God], who appeared in a fleshly body [was] vindicated [of false charges] by the Holy Spirit; [He was] seen by angels, was preached among the nations, was believed in throughout the world [and finally] was taken up into glory [i.e., heaven].

2 Timothy 2:17-18

and their talk will spread like gangrene. Among these people are Hymenaeus and Philetus,

Treasury of Scripture Knowledge

Summary

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

Bible References

The mystery

1 Timothy 3:16
And without question the revealed secret of godly living is great. [It is this]: He [i.e., Christ, or God], who appeared in a fleshly body [was] vindicated [of false charges] by the Holy Spirit; [He was] seen by angels, was preached among the nations, was believed in throughout the world [and finally] was taken up into glory [i.e., heaven].
Revelation 17:5
And on her forehead was written a mysterious [name], "The great Babylon, the mother of the prostitutes and the disgusting things of the earth."

Doth

Acts 20:29
[Because] I know that after I am gone [from here] there will be vicious wolves [i.e., ravaging false teachers] who will come in among you, and they will not spare the flock [i.e., the congregation].
Colossians 2:18
Do not allow anyone, who delights in acting humble [i.e., living an ascetic life] and who worships angels, to rob you of your reward [in heaven]. [Note: This "angel worship" could mean holding angels in high regard because of their role as God's messengers, or the claim that God can be approached only through angelic intermediaries]. He places great importance on what he [claims to] see [i.e., visions], being inflated with pride by his fleshly mind for no [good] reason.
2 Timothy 2:17
and their talk will spread like gangrene. Among these people are Hymenaeus and Philetus,
1 John 2:18
Dear children, it is [now] the last hour [i.e., the final period of God's dealing with people on earth], and since you have heard that an antichrist [i.e., a person who is against Christ] is coming, even now [you should be aware that] many antichrists have [already] come. This is [one way] we know that this is the last hour.
1 John 4:3
But every spirit who does not proclaim [this truth about] Jesus is not [speaking] from God. And this [person] represents the spirit of the antichrist [See 2:18-22; II John 7], whom you have heard would come. Well, now he is already here!

He who

2 Thessalonians 2:6
[But] for now, there is something holding him back [i.e., holding back the man of lawlessness from emerging, See verses 3-4], and you people know what it is. [It is restraining him] so that he will not be revealed until his own [due] time.