Brent mentioned the two conditions that must occur before The Day of the Lord, which would fulfill the apostle’s warning. There is actually a third: that the restraining force (also a man or perhaps of men) must be removed in order that the “son of perdition” may be revealed. So, there was to first come a falling away, then the “one who restrains” had to be removed, and finally the man of sin would be revealed.
I would suggest appeals outside of the text for consideration while examining the passage should at first be limited. We should stick first to the text without coloring it too much with passages from the prophets and the Old Testament until we have exhausted the possibilities offered by the direct text. I suggest this for two reasons: first, the Thessalonians likely knew exactly what Paul was driving at and why he had written to them, even if we do not. He was after all, responding to concerns some of them had raised. Second, as they were mostly neither Jews nor from a Hebrew background, it is unlikely that they knew much of anything about the Old Testament, the prophets or the foundations of Israel; and such things would not have meant anything to them or have “talked” to them. They had come out of a completely different background, culture, and set of circumstances. Even the trials they were suffering were home grown. The language of the prophets may have stirred them, but they likely would not have known the derivations.
The phrase “the Day of the Lord” is the identical phrase as was used in the first letter to Thessalonica. The apostle stated concerns about their understanding of the Day of the Lord, and here also mentions “our gathering together with Christ.” He can therefore only be referring to a single topic: that the Day of the Lord of his comment is the Judgment Day. I see no other choice in the text.
Additionally, the word “present” describing the nearness or coming of the Day rightly seems to have made its way from translators into all of the English margins or notes and appears to be proper and warranted. With these things noted, what he wanted them to know was that the Day of Judgment was not yet present, had not yet begun or was not “at hand.” It remained in the future, as did the signs of its commencement.
As Brent also mentioned, Paul noted that this Day must first be preceded by a rebellion, or literally – a falling away (the literal translation of the transliterated word apostasy), and by the revelation of “the man of sin” or “the son of perishing.” That first phrase, “the man of sin,” is preceded with a definitive article just as is “the Day of the Lord” in every text group: the Majority Text, the Received Text and the Critical Text. So both phrases are correctly rendered as “the Day of the Lord” and “the man of sin” or some correct variation of that.
It appears (to me anyway) that the letter was therefore dealing with teachings and markers that would have to occur to begin the cycle of the commencement to the end of time and the Judgment Day, and with a specific man or type who would be revealed in the midst or during a wholesale departure from the truth of the gospel and of the apostle’s doctrine, after a restraining force had been removed. The apostle was not putting a date or time stamp on things but was merely identifying that certain markers would have to be met before things would ever begin moving that direction. And departures from the gospel and the apostle’s doctrine are the only type of departures that matter.
Before proceeding, we ought to dispense with the notion that Paul was addressing those from our very remote past but far beyond the day in which he penned this and had sent it out; or that it is somehow written to us today or even to those yet into the future. Every bit of that negates all of the rules of composition from within the letter itself and from any time and for every language.
It simply had to have meant something specific to the audience to whom it was addressed, else why bother writing to them at all? To suggest that the apostle who so loved them and had worked so hard on their behalves was with this letter navigating around their questions and concerns and stonewalling them; and that in place of answers was offering up visions of a future that could not be theirs, with remote images they would never see, is beyond absurdity, wholly unreasonable, and completely unsustainable.
This letter is then embodied with sparse though pertinent information for the Christians in Thessalonica and elsewhere. It is necessary information to them particularly so that they could make legitimate present day decisions by identifying who was telling them the truth and who was not; so that they might not “soon be shaken whether in mind or in spirit.”