This passage has been the subject of much controversy, as to whether this speaks of a period of great "apostasy", or whether it refers to the "departure", or "Rapture" of the Church.
I believe the KJV phrase "falling away", in 2 Thess. 2:3, refers to the "departure", or "Rapture" of the Church.
The KJV English phrase "falling away", in 2 Thess. 2:3, is the one Greek word "apostasia", and its basic meaning is to "depart from" or "go away".
The Greek word "Apostasia" is a compound of two Greek words: "Apo" = "to move away", "stasis" means "standing or state", or "to stand".
Literally, from its basic definition, "apostasia" means "to go away from", or "depart", or "change state or standing from one state to another".
"Apostasia" was used in extra Biblical Greek literature to describe political revolt, or a "going away from the establishment" and in the Septuagint, or Greek Old Testament, when the Jews would "go away" from God to worship other gods.
"Apostasia" is only used one other time in the New Testament, in Acts 21:21 to describe "forsaking", or "going away from" the teachings of Moses.
"Apostasion", the noun form, appears in Matt 5:31, & 19:7, and Mark 10:4 where it describes a "writing of divorcement", or "papers that separate". (Again, so someone can go away).
"Apostasia" literally then means "to depart, or go away from", and to "go away from what" must be determined from the context.
So, what is the "context" of both the First and Second Epistle to the Church at Thessalonica? The sole subject and context of both epistles is the "Rapture", or "Departure" of the Church, or "the called out ones," and advice to the Church while we await the Return of Christ for the Church. The Second letter to the Church at Thessalonica appears to have been written by Paul to clear up misunderstandings about his First Letter. (2 Thess. 2:1-5)
The Greek word "apostasia" in 2 Thess. 2:3 also has the Greek article "the" in front of it, in the Greek text, which makes it, not a general "going away", or "departure", but "The Departure", a special EVENT, that the reader is expected to already know about.
In other words, the use of the article "the" with "apostasia" in 2 Thess. 2:3 indicates that Paul expects the Thessalonian Christians to already understand that this is the title of an event, and he expects them to already know what it means.
Had the Apostle Paul already taught the Thessalonians about an "EVENT" that could be described as a "departure", or "going away"? Absolutely, yes.
Paul had already taught the Thessalonian Church about the EVENT, of the Catching Away and "Departure" of the Church in 1 Thess 4:13-18.
In 2 Thess 2:5, Paul says don’t you remember? When I was with you I taught you about these things?
I don't see where Paul taught them at all about "a falling away from the truth" in his first letter, but he taught them about the Rapture of the church in at least five passages in 1st Thessalonians:
1. 1 Thess 1:10 "And to wait for his Son from heaven, whom he raised from the dead,[even] Jesus, which delivered us from the wrath to come."
2. 1 Thess 2:19 "For what [is] our hope, or joy, or crown of rejoicing? [Are] not even ye in the presence of our Lord Jesus Christ at his coming?"
3. 1 Thess 3:13 "To the end he may stablish your hearts unblameable in holiness before God, even our Father, at the coming of our Lord Jesus Christ with all his saints."
4. 1 Thess 4:13 - 5:10:
Verse 13 "But I would not have you to be ignorant, brethren, concerning them which are asleep, that ye sorrow not, even as others which have no hope.
14 For if we believe that Jesus died and rose again, even so them also which sleep in Jesus will God bring with him.
15 For this we say unto you by the word of the Lord, that we which are alive [and] remain unto the coming of the Lord shall not prevent them which are asleep.
16 For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first:
17 Then we which are alive [and] remain shall be CAUGHT UP together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.
18 Wherefore comfort one another with these words.
Quasar92