Apostasia -- 2 Thessalonians 2:1-8
“Now we beseech you, brethren, by the coming of our Lord Jesus Christ, and by our gathering together unto him, 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.”
2 Thessalonians 2:1-2
Here Paul is writing to the church at Thessalonica about "the coming of our Lord Jesus Christ" and "our gathering together unto him" (Ephesians 1:10), and finally he mentions "the day of Christ". He goes on to explain that certain things must happen before these can be fulfilled
“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 2:3
Here, he specifically mentions two things that must happen before "the day of Christ" will come: a falling away (Greek: apostasia) and that man of sin be revealed, the son of perdition. The word "apostosia" simply means "departure". Some have taken it to mean departure from the faith (e.g. 1 Timothy 4:1), others have taken it to mean departure of the church (i.e. being "caught away" 1 Thessalonians 4:17). So, it this is not clear whether it is talk about departure from the faith or is rather referring to the departure mentioned in verse 7 "until he be taken out of the way". But what we know for certain is that once something departs, the man of sin will be revealed.
“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. Remember ye not, that, when I was yet with you, I told you these things?”
2 Thessalonians 2:4-5
This seems to be talking about the "little horn" in Daniel 7:8 who "shall speak great words against the most High..." Daniel 7:25. This same one is again mentioned in Revelation 13:5-6.
“And now ye know what withholdeth that he might be revealed in his time. For the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way. 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:”
2 Thessalonians 2:6-8
Here we see "what" is withholding (or restraining, holding him back). But we see from verse 7 the that once "he" is removed, the wicked one will be revealed. What is restraining sin in the world? It seems to be referring to the Holy Spirit here, who is working in and through the church to convict the world of sin, righteousness, and eternal (John 16:8). Could the departure be this?