There are different views of the restrainer of 2 Thessalonians 2, and a case can be made for two at least. I haven’t given it sufficient study to come to a firm conclusion, and I expect if I did I would still be uncertain. However:

1) The standard pretrib understanding of 2 Thessalonians 2 is presented in chapter 5 of Gerald B. Stanton’s book Kept from the Hour, titled Who Is “the Restrainer”?

2) The following are excerpts from the article Unmasking the Restrainer, which presents a different understanding.

But before we identify this restrainer, let us first determine that which the restrainer is holding back. Ernest Best pointed out in his list of “certain facts” we know about the restrainer that the participle has no compliment; that is, no direct object to receive the action of the participle. The direct object is implied rather than explicitly stated. So we must supply that direct object.

Nearly all of the popular solutions to the problem of the restrainer’s identity have assumed that the man of lawlessness is being held back by the restrainer. This has always been the premise of previous studies. Until now, it seems, no one has seriously questioned this assumption. But is this, in fact, the correct implied direct object of the participle? We must be willing to explore every possibility in order to be assured that we have arrived at the correct solution.

So, perhaps the correct question to ask of the text is not, “What is holding back the revelation of the lawless one?,” but rather, “What is the restrainer holding back?” To answer this more foundational question, let us go back to the subject Paul is addressing. It is essential that we interpret this in view of the wider context. Paul clearly states his topic in verse 1: “Concerning the coming of our Lord Jesus Christ and our being gathered to him.” The whole of chapter 2, then, is the Pauline response to the confusion about the arrival of this historic event. The coming of our Lord is what Paul is concerned with in this passage, not the coming of the lawless one. The confusion of the Thessalonians was not about the coming of that lawless one, but over the rumor regarding the arrival of the day of Yahweh. Accordingly, Paul is explaining that the Messiah’s parousia has not yet occurred because, in fact, other prophesied events must be fulfilled first. So, “the coming of Christ and our gathering to him” are being restrained. Thus, verse 6 should be understood to say, “and now you know what is holding him (Christ) back,” or even, “now you know what is holding it (the day of Yahweh) back.”

The apostle corrected any misunderstanding about the Lord’s coming in verses 3 and 4 by reminding them of what he had explained at length while he was with them - that the day of the Messiah’s parousia can not arrive until the prophetic events of the tribulation period are fulfilled. Then in verse 6, Paul reiterates his correction to their confusion when he begins: “And now you know what is holding...back.” So what is it that the Thessalonians now know? They were just told and now know that the day of Yahweh will not come until after the apostasy occurs and the man of lawlessness is revealed. Those two events were standing in the way of the parousia of the Messiah and the rapture.

The popular solutions to the identity of the restrainer assume that the man of lawlessness is being held back. But why would Paul be discussing what is holding back the lawless one when the more pertinent information the Thessalonians needed to be reminded of was the fact that certain prophesied events were holding back the coming of the Lord? The whole point Paul is driving home in chapter 2 is that the day of the Messiah’s parousia is being hindered! The restrainer, therefore, is holding back the day of the Messiah’s posttrib coming and the rapture of the church. Since the day of Yahweh is being held back, then, as we have already discussed at length, the apostasy and the revelation of the man of lawlessness are holding it back, because those two events must occur first. The restrainer, then, is the apostasy and the revelation of the lawless one.

Later excerpt:

As we mentioned before, most English versions of the Bible translate 2 Thessalonians 2:7,8a alike. The NIV says that “the one who holds it back will continue to do so till he is taken out of the way. And then the lawless one will be revealed....” Because of this kind of rendering, nearly all who read this text are led to believe that only when a certain individual is removed will the lawless one be revealed. But we have already shown that the restrainer is not holding back the revelation of the lawless one. Instead, the lawless one is that restrainer who is holding back the day of Yahweh. The mistranslation of the phrase “taken out of the way” is a major reason for the confusion about the restrainer’s identity.

First, the word translated, “taken,” is not an accurate rendering of the Greek word in our text. It was pointed out earlier that in order to do justice to this passage of Scripture, we must insist on utilizing the normal meaning of words as long as the normal meaning renders a coherent thought and makes sense within the context of what the author is writing. Unfortunately, in 2 Thessalonians 2:7, the normal meaning of the word translated “taken” was not used. Consequently, the translation not only misses the point of what Paul was saying, but it renders the exact opposite sense which he intended.