Charles C. Ryrie
The Liddell & Scott Greek-English Lexicon carries two definitions for "apostasia." You can easily find out about this very acclaimed lexicon on the internet. In this very acclaimed lexicon are apostasia's top-two definitions: definition #1 is "defection; revolt," and definition #2 is "departure; disappearance." "Departure" in Greek relates to physical departure or in English, disappearance. Therefore, it fits if describing what occurs in the rapture. Those same two definitions for apostasia appear in various articles, always citing Liddell & Scott.
I'm not making up anything about 2 Th 2:1-3. The early Bibles say (see at bottom) what I am trying to get you to understand. 2 Th 2:1 merely raises the topic of the rapture. Paul needed to address the rapture because he got word that his Thessalonian flock had received a fraudulent letter, made to appear to be from him, saying that they had missed the rapture and were in the Trib. All that is directly or indirectly in 2 Th 2:2-3. I am not making anything up. In verse 3, Paul corrects his flock: "That day" (referring to verse 2's "Day of the Lord") will not come as the apostasia comes first. In this context of rapture, only definition #2 of apostasia fits.
Recap: in verse 1, Paul cites the rapture, therefore the context is rapture. In verse 2 and in early verse 3, he acknowledges the deception suffered by his flock. In the rest of verse 3, he says the Trib (Day of the Lord) won't come as the apostasia (departure; disappearance) comes first. We know it's definition #2 of apostasia because the context is rapture.
Here are three verses from early Bibles, covering 2 Th 2:1-3. They say what I claim about them. I am not making anything up.
2 Th 2:1 (Geneva Bible): Now we beseech you, brethren, by the coming of our Lord Jesus Christ, and by our assembling unto him,
2 Th 2:2 (NLT): Don’t be so easily shaken or alarmed by those who say that the day of the Lord has already begun. Don’t believe them, even if they claim to have had a spiritual vision, a revelation, or a letter supposedly from us.
2 Th 2:3 (Geneva Bible): Let no man deceive you by any means: for that day shall not come, except there come a departing first, and that that man of sin be disclosed, even the son of perdition.
This is a verse that was written about what happened in Thessalonica after Paul left, after teaching about the rapture (1 Th 1:10 and 1 Th 4:16-17). This is what led to the fraudulent letter being sent to Paul's Thessalonian flock: Acts 17:5 (NIV): But other Jews were jealous; so they rounded up some bad characters from the marketplace, formed a mob and started a riot in the city. They rushed to Jason’s house in search of Paul and Silas in order to bring them out to the crowd.