The Argument as presented by the pre-Tribulation camp as follows –The word can mean a separation from a previous state a one time event having nothing to do with faith; it is translated Divorce three times in scripture. The argument states that the understanding of the word is based on the context in which it is used. So the argument sees the Rapture in the context of the 2 Thessalonians because it is tied to The Day of The Lord passage and therefore The rapture is the one time event or Separation which precedes antichrist and the day of the Lord in 2 Thessalonians 2
First let me say that this is an obvious and confusing twist to the actual meaning that they are applying to the word apostasy in this passage. Everything else is based on the definition which allows one to think that they might be correct.
However when realizing that what Paul is trying to clear up is confusion about the day of the Lord then we see that the focus is not on the Rapture. What Dispensationalist’s are trying to do with “apostasy” of course can only be done if the Rapture is tied directly to the day of the Lord, but this in reality is not the case when applying the pre-Trib thinking. Why, because their rapture teaching does not tie the Rapture itself to the “Bible’s” definition of the Day of the Lord. Their teaching places it at least 7 years prior. But Paul’s own writing to these same Thessalonians in 1 Thess 5 does make the context about the Day of the Lord, and yes it includes the Rapture, but not for the reason the Dispensationalist has concluded.
The argument presented in this passage by pre-Trib scholars is that Jesus coming as a thief is the Rapture and His coming in the clouds (clouds being the Great army of saints) is His Glorious Appearing to Earth 7 years later when every eye will see Him.
We know however that the Glorious appearing occurs on the Day of the Lord and begins the Millennium, why? — Joel tells us that the earth will be in total darkness before the day of the Lord. Listen:
behold, I am going to arouse them from the place where you have sold them, and return your recompense on your head. Proclaim this among the nations: Prepare a war; rouse the mighty men! Let all the soldiers draw near, let them come up! Beat your plowshares into swords And your pruning hooks into spears; Let the weak say, “I am a mighty man.” Hasten and come, all you surrounding nations, And gather yourselves there. Bring down, O Lord, Your mighty ones. Let the nations be aroused And come up to the valley of Jehoshaphat, For there I will sit to judge All the surrounding nations. Put in the sickle, for the harvest is ripe. Come, tread, for the wine press is full; The vats overflow, for their wickedness is great. Multitudes, multitudes in the valley of decision! For the day of the Lord is near in the valley of decision. The sun and moon grow dark And the stars lose their brightness. The Lord roars from Zion And utters His voice from Jerusalem, And the heavens and the earth tremble. But the Lord is a refuge for His people And a stronghold to the sons of Israel.
Joel 3:7,9-16 NASB
DARKNESS and ARMAGEDDON; the harvest is ripe and the wine press full none of these are unfamiliar characteristics of the prophetic signs concerning Tribulation before THE DAY OF THE LORD, these are stated to be the conditions just before the Lord returns in Isaiah, Revelation and in Jesus’ accountings. So Paul says in 1 Thess 5, that His coming as a thief IN THE NIGHT is on the Day of the Lord, making this His coming after Tribulation even as Jesus described it in Matthew 24 and Mark 13. Paul here says as a thief in the night. Darkness which precedes this day is seen here by Paul as night. Making the Rapture and Second Coming one event as are the resurrection and Rapture that he had just described in the preceding verses in chapter 4.
So the pre-Trib argument is not valid because it says that the day of the Lord begins at the beginning of the Tribulation; yet Revelation, Joel and Isaiah all specifically end the Tribulation with Armageddon and darkness over the earth. So today there is now an attempt to overcome this newly realized problem, because in the distant past the pre-Trib dispensationalist would state that the tribulation is definitely a part of the day of the Lord. Also today a valid objection is raised that by so doing they are also refuted by PAUL where he says in 2Thess that the Day of the Lord will not come until the Apostasy and the Antichrist come. So since Jesus places both of these events as occurring during Tribulation their argument has now shifted.
Note this prior argument has not been abandoned, because people still believe it. Why?— very few are interested in or are willing to share an article such as this one. So many still make these same arguments with no Scriptural support, but the focus surrounding these verses has changed for many, and it now is about the meaning of the word Apostasy. But it goes without saying that trying to make Rapture out of Divorce or falling away, or even removing oneself is a big STRETCH. If we go back to the definition of the word apostasy the actor is the one who apostatizes. So here the church is the actor who falls away or removes itself from the earth. The ordinary meaning would be that one would fall from the faith, but here it is all of the church removing itself from the earth. Can the church remove itself? What would that do to this scripture?
I write these articles and expect those few who read them to share them if you consider them worthy. Very little of that is being done and so the popular message, to escape a Tribulation that will surely come, is still the theme of the day. My thinking is that this condition itself will be the actual catalyst for the falling away from the faith that will come in these last days — not because I said it, but because the scripture says it. Jesus said it.
Finally, please forgive me for using Imminence in this title. I know that it is really not the subject, but it certainly is a big theme within pretribulationism’s argument today. As for my use I’m just trying to attract as many as I can to what scripture really says. Thank you for reading.
Also see the following article also posted earlier on this site for more on this topic: Apostasy — to fall away; to depart. Does this departing reference the Rapture?
Short Definition: defection, apostasy Definition: defection, apostasy, revolt.
646 apostasía (from 868/aphístēmi, “leave, depart,” which is derived from 575/apó, “away from” and 2476/histémi, “stand”) – properly, departure (implying desertion); apostasy – literally, “a leaving, from a previous standing
Use of a perfective aspect, however, does not imply a punctiliar or short-lived action. It simply “presents an occurrence in summary, viewed as a whole from the outside, without regard for the internal make-up of the occurrence.” 
I know that there is a lot there, but you may want to reference it.