I did give an answer to that question, but someone simply deleted it, even while admitting it was a well though-through answer based on Scripture. I'd reckon some just don't want to know.
As far as I can tell, people rather see tradition being posted here as an answer (things that "saints" said), instead of going to what Scripture itself says. Anyone thinking that is not the case, by all means comment below.
Admittedly, I did find it a bit grievous, and still do, but perhaps the mature option would really be to just let it rest. Still, I'd rather have a number of downvotes and comments on it, for people to show themselves as they are up-front. I agree that mods need to exercise some quality control, but for a "NAA" to be handed out, I'd think something has to be really off-topic or out of bounds, such as argumentative specifically against a person. If Scripture does not support an idea, than that idea is not Christian in origin and I don't see why that can't be acknowledged openly, without telling people what to believe. That latter is for each to decide on their own, but at least come out about it as it is, without making a pretense.
Here's the post that another answer figures I had deleted:
This has become a lengthy post due to the amount of Scriptural evidence discussed here. The ultimate answer to the question is at the end in bold italics. If it doesn't seem right, first read the discussion please, then comment.
Paul was mentioning an apostacy in verse 3, meaning a deviation from the truth that was being taught at that point. In verse 4 he points out that this 'man of lawlessness' will sit down "in the temple of God, publicly showing himself to be a god". He also mentions in verse 7 that "the mystery of this lawlessness is already at work, but only until the one who is right now acting as a restraint is out of the way". Then he goes on to remind them that, after the restraint had gone, the lawlessness would not stop, but continue out in the open.
Is Paul saying something new? No, by no means he is, because it was already known that these things were 'brooding', so to speak. There are a number of references in the Greek Scriptures. First of all, Jesus referred to false prophets rising up:
“Go in through the narrow gate, because broad is the gate and spacious is the road leading off into destruction, and many are going in through it; whereas narrow is the gate and cramped the road leading off into life, and few are finding it.
“Be on the watch for the false prophets who come to you in sheep’s covering, but inside they are ravenous wolves. By their fruits you will recognize them. Never do people gather grapes from thorns or figs from thistles, do they? Likewise, every good tree produces fine fruit, but every rotten tree produces worthless fruit. A good tree cannot bear worthless fruit, nor can a rotten tree produce fine fruit. Every tree not producing fine fruit is cut down and thrown into the fire. Really, then, by their fruits you will recognize those men. ~Matthew 7:13-20
In answer Jesus said to them: “Look out that nobody misleads you, for many will come on the basis of my name, saying, ‘I am the Christ,’ and will mislead many. ~Matthew 24:4,5
Note the first of Jesus' statements that I included, which says something about the numbers: only a minority will find the road, or lifestyle, leading off into life. Compare that with 2 Thessalonians 2:4,8 where it says that the lawless one will sit down in God's temple, openly showing himself to be a god. That means that the large majority will be accepting of this one, so that he is not expelled, but rather changes the course (teachings) of the congregation (Christianity as a whole religion, you could say). This suggestion is supported by what Paul says in verse 11 that God will allow the deluding influence to exist, and that it will only done away with by Jesus (verse 8). This is further revealed in Revelation 18, where there is an urgent call:
And I heard another voice out of heaven say: “Get out of her, my people, if you do not want to share with her in her sins, and if you do not want to receive part of her plagues. ~Revelation 18:4
This "Babylon the Great" is identified in Revelation 17 as a "prostitute", just like in the Hebrew Scriptures, there are mentions of spiritual prostitution. In Revelation 17:15 it is revealed that this "prostitute" is something or someone that has to do with a large number of different people and nations and language groups; they all have something in common in this respect. Further, in Revelation 18, it is revealed that this one also gets involved with the "kings of the earth" (Revelation 18:9) and "the merchants of the earth" (Revelation 18:11), which indicates that this Babylon the Great is heavily involved with politics and commercial activities worldwide. The result? Revelation 18:3 says that she lives in "shameless" luxury; so this one is extremely wealthy. Finally, in Revelation 17:6, it is also said that this woman is "drunk with the blood of the holy ones and with the blood of the witnesses of Jesus"; and thus may be said to have been involved in the death of many of God's people. Moreover, Revelation 18:24 says that this one is responsible for all the killing that has been going on on the earth. A bonus: verse 23 even mentions marriages being performed 'in there'.
So, if you take all these hints together, that "Babylon the Great ..."
... is something people and nations worldwide have in common;
... is heavily involved with politics;
... is actively involved with commercial business;
... is extremely wealthy;
... is responsible for all the "slaughtering" on the earth; and
... is something that or someone who is involved in performing marriages.
Is there anything you know of that fits all of these descriptions? Would it be unreasonable to consider organised religion?
If that is the case, does it mean that all organised religion bad? That is not possible, because Hebrews 10:24,25 basically commands God's people to continue to meet together regularly, which has always been the case. The book of Acts and the apostolic letters also indicate that the early Christians were well organised. Jesus' words in John 4:23,24 are very helpful, where he mentions that there will be true worshippers. Now if there are true worshippers, that implies there are also false worshippers. Remember that in 2 Thessalonians 2:11, it was mentioned that God allows an amount of worshippers to be mislead, "so that they may come to believe the lie". With that adjustment, we can conclude that the six attributes above belong to what some call the world(wide) empire of false religion. I think the details of the 'misleading' part here are a bit out of scope, so I'll put that aside for now.
Who or what is the restraint?
In the days that Paul wrote the words in the question's text, there was already sufficient awareness that there was a force moving to draw the congregations away from the truth. These are some of the other references we can find in the inspired Christian Greek Scriptures themselves:
I know that after my going away oppressive wolves will enter in among you and will not treat the flock with tenderness, and from among you yourselves men will rise and speak twisted things to draw away the disciples after themselves. ~Acts 20:29,30
However, the inspired word clearly says that in later times some will fall away from the faith, paying attention to misleading inspired statements and teachings of demons, by means of the hypocrisy of men who speak lies, whose conscience is seared as with a branding iron. ~1 Timothy 4:1,2
Beloved ones, do not believe every inspired statement, but test the inspired statements to see whether they originate with God, for many false prophets have gone out into the world. ~1 John 4:1 (highlight mine: suggests something was already going on at that time).
My reason is that certain men have slipped in among you who were long ago appointed to this judgment by the Scriptures; they are ungodly men who turn the undeserved kindness of our God into an excuse for brazen conduct and who prove false to our only owner and Lord, Jesus Christ. [10-13] Too bad for them, for they have followed the path of Cain and have rushed into the erroneous course of Baʹlaam for reward, and they have perished in the rebellious talk of Korʹah! These are the rocks hidden below water at your love feasts while they feast with you, shepherds who feed themselves without fear; waterless clouds carried here and there by the wind; fruitless trees in late autumn, having died twice and having been uprooted; wild waves of the sea that cast up the foam of their own shame; stars with no set course, for which the blackest darkness stands reserved forever. [17-19] As for you, beloved ones, call to mind the sayings that have been previously spoken by the apostles of our Lord Jesus Christ, how they used to say to you: “In the last time there will be ridiculers, following their own desires for ungodly things.” These are the ones who cause divisions, animalistic men, not having spirituality. ~Jude 4,11-13,17-19
Several apostles mentioned that these things were and were going to happen. At the point that the letter of Jude was written, these things had already begun to happen. Compare how the wording of Jude echoes the warning that the apostle Peter gave in his second letter:
(...) as there will also be false teachers among you. These will quietly bring in destructive sects, and they will even disown the owner who bought them, bringing speedy destruction upon themselves. Furthermore, many will follow their brazen conduct, and because of them the way of the truth will be spoken of abusively. Also, they will greedily exploit you with counterfeit words. But their judgment, decided long ago, is not moving slowly, and their destruction is not sleeping. (...) [12-19] But these men, like unreasoning animals that act on instinct and are born to be caught and destroyed, speak abusively about things of which they are ignorant. They will suffer destruction brought on by their own destructive course, suffering harm as their reward for their own harmful course.
They consider it pleasurable to indulge in luxurious living, even in the daytime. They are spots and blemishes who revel in their deceptive teachings while feasting together with you. Their eyes are full of adultery and are unable to desist from sin, and they entice unstable ones. They have a heart trained in greed. They are accursed children. Abandoning the straight path, they have been led astray. They have followed the path of Baʹlaam the son of Beʹor, who loved the reward of wrongdoing, but was reproved for his own violation of what was right. A voiceless beast of burden speaking with a human voice hindered the prophet’s mad course.
These are waterless springs and mists driven by a violent storm, and the blackest darkness has been reserved for them. They make high-sounding statements that are empty. By appealing to the desires of the flesh and with acts of brazen conduct, they entice people who have just escaped from those who live in error. While they are promising them freedom, they themselves are slaves of corruption; (...)" ~2 Peter 2:1-3,11-19a
Peter makes some statements about how these people will be living: "they will greedily exploit you with counterfeit words" for one. These words, of course, will appear genuine, just as a scammer would want to seem genuine to succeed in getting your personal (bank) details to make money off you while giving nothing in return. They would be living a sinful, unapproved lifestyle (yet while wanting to appear approved) among the true Christians. They use complicated language (for example, using philosophical terms and phrases; compare Colossians 2:8: Scriptures opposed to philosophy and traditions) and they "consider it pleasurable to indulge in luxurious living, even in the daytime", which makes it clear that they would be obviously wealthy. Do you see the link with the description of the shameless luxury that "Babylon the Great" would be living in, as mentioned above (Revelation 18:3)?
Further on the warning that the false teachers would be rise up from among the true Christians, some may remember the parable that Jesus gave of the wheat and the weeds (Matthew 13:24-30). This illustration indicates that the time of the apostacy would continue for some time, so that the distinction between true and false Christians would become clear. This is in line with the prophecy in Malachi 3:18: "And you will again see the distinction between a righteous person and a wicked person, between one serving God and one not serving him".
Now as for the restraing, in Acts 20:29,30 Paul was speaking to the elders of the congregation of Ephesus and said that "oppressive wolves" would come in among them. This on top of the other warnings, makes it seem that the apostles were acting as restraints. This makes sense, because of having been eye-witnesses of Jesus' time on earth and hearing him teach first-hand, learning from him directly, the apostles had a measure of authority; even if only because of that. So to find out which Christians are true and which ones are not, you'd have to compare with how the first congregations worked and worshipped. So, for example, in Acts 15 you can read the account of how a dispute was resolved: a body of elders and apostles gathered together, discussed the Scriptures, and reached a conclusion. That leads to the conclusion that there is no single human leader in true Christianity, as Jesus pointed out also: "all of you are brothers" (Matthew 23:8-12).
Concluding anwer to the question
Before anything else, we cannot judge or even fully know the motives of any individual, but from the above we can get a general idea of what leads towards such things being said:
***Least bad scenario would make it a matter of lacking knowledge and understanding. The worst case would be ignorance (as in not wanting to know) and wilfully misleading others into believing whatever they say. So you are quite right to conclude that these two conditions were to be met first, but it seems that some of the recipients were still alive when they great apostacy started to make an appearance.
The reason Paul was 'reassuring' the congregation(s) was, as he refers to in verse 2, there would be people standing up to claim that they were the (possibly returned) Christ, or claiming that they were speaking in behalf of the apostles, with the message that "the day of Jehovah" had arrived. Such claims would be unfounded and possibly move the Christians to do or expect things that they shouldn't. He finishes encouraging them to "maintain hold on the traditions that you were taught".
So Paul did not teach anything like a pre-trubulational calling to heaven.***
Is there anything that can be done? Yes, on your part, take to heart the following counsel by not blindly believing what religious leaders say, but studying the Scriptures for yourself, in prayer as God can give understanding (1 Corinthians 2:10-16):
"The naive (or: "inexperienced") person believes every word, but the shrewd one ponders each step." ~Proverbs 14:15
"Immediately by night the brothers sent both Paul and Silas to Be·roeʹa (...) Now these [Jews] were more noble-minded than those in Thes·sa·lo·niʹca, for they accepted the word with the greatest eagerness of mind, carefully examining the Scriptures daily to see whether these things were so." ~Acts 17:10,11
In addition to that, accept help that explains Scriptures by using the Scriptures, not through the sayings of highly esteemed figures that turned up after the time that the Christian Greek Scriptures were completed (with Revelation being the last one), just as the Ethiopian Eunich accepted help (Acts 8:26-39). And do not be misled, if even this intelligent "minister of finance" of a substantial empire (verse 27) needed assistance to understand what he was reading, then we will most probably need some assistance as well.
Edit based on comment: The apostles obviously supported the ideas as indicated by their writings, as did at least many of the 1st century Christians. That is sufficient to define proper doctrine. Opinions of (groups of) individuals later in time, however popular they are or might have been, is perhaps preferred, but certainly not required.