From what I gather TDW, you interpret what's translated as "falling away" (Apostasia) as "the rapture"...
"
he apostasia / he apostasis" ["
THE Departure"] referring BACK to what has ALREADY been mentioned in the text (not further down in the text)... "A STANDING away-from ['a standing away-from' a previous standing]"... but in this case, WITH the "definite article ['THE']"... so a PARTICULAR one (the one
previously mentioned in the text), meaning [at its most basic meaning] "departure" [see also "
stasis/stasin" as used in Heb9:
8-9a]
and that the strong delusion Paul predicts is its modern translation as "falling away"?
Am I reading your post correctly?
Not what I'm saying.
100% of "the Church WHICH IS HIS BODY" (ALL those saved "in the present age [singular]") will go in the Rapture (even the "Corinthians"

)...
This leaves 100% of the remaining people on the earth (at the START of the 7 Trib years) being "unbelievers/unsaved persons"... including a good portion of [what we commonly call] "Christendom" (not 100% of "Christendom" is SAVED, see).
Once "our Rapture" takes place, ["poof! "], there will be many more ppl
who will be coming to faith, as well as
many ppl who will NOT but who will "believe the LIE/the FALSE/the PSEUDEI" INSTEAD (God will have sent
THEM [
at some point FOLLOWING "our Rapture"] "
strong delusion THAT they should BELIEVE [it]")... and Paul is telling of
this CONTRAST (
of TWO DISTINCT "beliefs" ppl WILL come to, in the trib years, as he refers to this time-period
at either end of his "bookended" CONTEXT [chpts 1 & 2], where the phrases "the
DOTL [
earthly time-period]" and "IN
THAT DAY" speak of the
SAME TIME-PERIOD [
just as they do in ALL contexts where these two phrases
are found in close proximity in the text, as they are here also]).
Now, try to imagine yourself (
for illustration, for the purpose of
trying to picture this [AS SUCH a person]) in the place of one of those
unsaved persons finding themselves on an earth where "lots and lots of ppl"
have just disappeared/vanished (
even if you [/they] are not "cognizant" that you are now EXISTING IN / EXPERIENCING [what WE call] the Tribulation Period [7-yr]); and let's say, the neighbor to your right is "Jewish" [who happened to be UNSAVED], the neighbor to your left is one of those from "Christendom" [but who ALSO were not actually SAVED], and only ONE neighbor, way down the street who you really only knew
in passing/vaguely is among those (all over the world) who "vanished/disappeared"...
and you yourself [
not "you" literally
]
only have a VERY FUZZY awareness of what others have said "about the Bible," but
in such a grossly-limited way that, if you ever even start to *wonder* if "
God" had anything to do with "
what just happened," you wouldn't be able to really piece it all together
in a sharply-accurate way all of a sudden...
so (since you are aware that the "disappearances" were WORLD-WIDE, not just your ONE neighbor down the street),
...and since your whole neighborhood is out on the block talking about this strange thing that's happened recently... (
and *IF* you have ANY inkling that "GOD" was at back of this "happening"), naturally you might be inclined to actually approach each of these two particular neighbors
who "might have some idea what the BIBLE has to say" about such "happenings"... and so the convo begins [with these two "
UNSAVED-but-at-least-
acquainted-with-the-Bible" persons...]... Imagine such a scenario... and try to envision what they might each have to say about it, to you. The Jewish man never believed "
Jesus" was the Messiah, and the "Christendom"-person never
relied on Him for salvation (
otherwise, they wouldn't still be here, following "our Rapture"), but they each have a "take" on the Bible and "what it says" (
since this is the Subject you've brought up in their presence, for the purpose of ascertaining... something..., IF there's such a thing on it, in there...).
ENTER the fact that God has already told us that "HE SHALL SEND *them* [
particular ones HE KNOWS the hearts of--but not to 100% of those on the earth following "our Rapture" 
] strong delusion THAT they should believe THE LIE/the FALSE/the
PSEUDEI," and let's just say (for ILLUSTRATION PURPOSES *ONLY*) that it is one of these neighbors [take your pick, it doesn't matter for this *illustration* (we'll have the OTHER one be the one who "comes to faith" a bit later on during these trib yrs--again,
illustration 
)]... the one particular neighbor being to whom GOD
SHALL SEND this "strong delusion,"
and thus they begin to educate you on [what they consider to be] "the facts of the matter" (concerning "what just happened" and how [
in THEIR view] "God" was involved--
after all, you DID *ask* them... and so they respond...)...
... I'm going to close this post now, and just let you try to "picture" such a scenario, and "where that might go" (as far as WHAT "the LIE/the FALSE/the
PSEUDEI"
might consist of and entail...)
____________
[I gotta go hit the pillow! Night all! = ) ]