After having covered how "apostacy" is a major theme of Matt 24 at http://www.fivedoves.com/letters/march2013/jovial311-1.htm, lets examine how this is also the case in 2 Thess 2:3.
A lot of pre-trib rapture believers like to explain 2 Thess 2:3 away by saying that the word "apostacy" in 2 Thess 2:3 means "rapture", and not an "apostacy". At least one problem with that argument is that the entire context of the text is discussing rebellion / apostacy. Let's read through 2 Thess 2:2-7, and I will put my comments to the right to indicate how much the entire chapter is talking about the concept of apostacy.
Verse (in 2 Thess Chapter 2)
2be 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 This describes "apostating" from an emotional state of peace. 3alet no many deceive you Deception involves a rebellion / apostacy against the truth. 3bfor that day shall not come except there come an APOSTACY (apostacia) first Yes, this is talking about an apostacy, not the "rapture" 3cand the MAN OF SIN (LAWLESSNESS) be revealed This is the only place he is called the "man of sin", a term that describes him as an apostate, or a rebel, in rebellion against G-d. The word "SIN" here is actually LAWLESSNESS - refusal to accept G-d's Law. 3dthe SON OF PERDITION again, another term that describes him as an apostate is used here. Judas was called this in John/Yochanan 17:12. Judas rebelled or apostated from the 12 chosen. 4Who 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 Once again, apostacy is the theme of this verse.
6 And now ye know what withholdeth that he might be revealed in his time.
Much like Matt 24 developes the concept of rebellion or apostacy into different forms, so does this chapter. Restrainment is an apostacy of sorts by something aganist something else. The MAN OF LAWLESSNESS want to reveal himself, but something is apostating against his ability to do that.
7 For the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way.
Again, apostacy or rebellion is the theme of this verse, saying that the power of apostacy (or mystery of iniquity) is already working in the world. But something else is restraining it, and again, restrainment is an apostacy by something against soemthing else.
So similar to Matthew 24, apostacy is a theme that is discussed multiple times over and over again in 2 Thess chapter 2 and developed as an evolving theme, with different shades of ideas similar to apostacy coming into and out of play as the discussion progresses. But how can there be an argument that "apostacy" here is not talking about apostacy when there is so much content in the context supporting that as the appropriate interpretation of the verse?
Unless you just WANT this verse to mean "rapture", you can't get that out of the context. We must fit our theology to the written text, not the text to our desired theology.