The Apostasy Paul specifically spoke about has not yet happened.
2Th 2:1 Now we beseech you, brethren, by the coming of our Lord Jesus Christ, and by our gathering together unto him,
2Th 2:2 That ye be not soon shaken in mind, or be troubled, neither by spirit, nor by word, nor by letter as from us, as that the day of Christ is at hand.
2Th 2:3 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;
2Th 2:4 Who opposeth and exalteth himself above all that is called God, or that is worshipped; so that he as God sitteth in the temple of God, shewing himself that he is God.
That is the Antichrist! Claiming to be God causes the Apostasy or departing from the Christian faith.
2Th 2:5 Remember ye not, that, when I was yet with you, I told you these things?
2Th 2:6 And now ye know what withholdeth that he might be revealed in his time.
2Th 2:7 For the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way.
2Th 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:
The context is regarding the timing of the return of Christ. That cannot happen until two important things occur.
1. The falling away/Apostasy
2. and the revealing of the Antichrist
Obviously it is his appearance which causes the falling away because they fall away from Christ to him, the AC or false christ.
So it hasn't happened and isn't happening now. The AC must appear which will be a global event, then those who had followed Christ will follow the AC. Why? IMO he will claim to be Christ so they won't realize they have fallen away from Christ. This happened during the tribulation and as Christ states in Matthew, the second coming is not until the trib has ended.
Why did Paul even write this? Because people were concerned Jesus could just return at any moment without any warning (like a thief in the night) and Paul wanted to clarify that certain events had to happen first. Paul accidentally started the notion of the second coming happening at any time which is the basis for the pre-trib doctrine.
Contextually Paul was not speaking of a long period of falling away. It was a special event where people would stop being Christians and have a new God to worship and it is tied directly to the tribulation and the AC claiming to be God! Clearly it is when people are deceived and begin worshiping the AC because they think he is God. I believe many Christians will think he is Jesus Christ and won't realize they committed Apostasy.
2Th 2:3 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;
2Th 2:4 Who opposeth and exalteth himself above all that is called God, or that is worshipped; so that he as God sitteth in the temple of God, shewing himself that he is God.
This has not yet happened but it is coming. Remember that the falling away is directly related to someone claiming to be God. They "fall away" to that false god.
Remember Christ's warning:
Mat 24:23 Then if any man shall say unto you, Lo, here is Christ, or there; believe it not.
Mat 24:24 For there shall arise false Christs, and false prophets, and shall shew great signs and wonders; insomuch that, if it were possible, they shall deceive the very elect.
Mat 24:25 Behold, I have told you before.
Mat 24:26 Wherefore if they shall say unto you, Behold, he is in the desert; go not forth: behold, he is in the secret chambers; believe it not.
There will come a time when people will believe Jesus has returned but it will be a false Christ.
1Ti 4:1 Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils;
Here the Apostasy is said to happen in the "latter times" as opposed to over a long span of time. This places it within the tribulation which also is a certain period of time just before the second coming.
2 Thessalonians 2:3 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;
2 Thessalonians 2:4 Who opposeth and exalteth himself above all that is called God, or that is worshipped; so that he as God sitteth in the temple of God, shewing himself that he is God.
If I may put it in my own words:
Let no man deceive you by any means!
The return of Jesus Christ shall not come until there is a falling away from Him by those who were once Christians.
After that great defection to another Christ, the sinful man who claimed to be God will be uncovered and exposed as having been the son of perdition all along. This happens when the true Christ returns!
This sinful man who claimed to be God but was actually the son of perdition in disguise who opposed and exalted himself above all that is called God, or that is worshipped; so that he as God sat in the temple of God, shewing himself that he was God. He was worshiped as God and that's how he caused the great falling away.