Paul: Concerning the rapture [1], don't be persuaded by any false conveyors claiming 'that the day of the Lord is already here [perfect indicative][2], ... because it will NOT be [present][2], if not shall have come THE DEPARTURE/OUR episynagoges UNTO HIM/our Rapture IN THE AIR *FIRST* [1--the Subject Paul BROUGHT UP in v.1], and the man of sin be revealed...
Thus, two evidences must be in play for such a claim to be true (neither of them are--therefore is a "false claim"), but only ONE of them is said to be *FIRST* (not BOTH of them are *FIRST*, as some mistakenly read this text to be saying).
Subjects:
[1]--"OUR episynagoges UNTO HIM" (our Rapture--where we go TO "the meeting of the Lord IN THE AIR]"
[2]--the false claim "that the day of the Lord" earthly-located time-period of "JUDGMENTs" unfolding upon the earth over SOME TIME (the "IN THE NIGHT"/"DARK"/"DARKNESS" time-period) is already here... and Paul's words instructing them not to believe that false claim that it IS ALREADY HERE
[3]--3a [the "WHY"] "because NOT"... "because it will NOT be present [Subject 2], if...
------3b "if not shall have come THE DEPARTURE *FIRST* [Subject 1]...
------3c "AND the man of sin be revealed..." [belonging to Subject 2--in "the day of the Lord" earthly time period of JUDGMENTS unfolding, when the "man of sin" will have "ARRIVED" on the scene per v.9a to DO ALL he is slated to DO over the course of those "7 years"--That is, when he is "revealed," that is when "the day of the Lord" time period WILL INDEED be present, and which time period's ARRIVAL is LIKE the INITIAL "birth PANG" that COMES UPON a woman... NOT when they've ENDED, see... (and which Jesus Himself had also spoken of in His Olivet Discourse and which are equivalent to the SEALS of Rev6 at the START of the "IN QUICKNESS [NOUN] time period, aka 7-yrs, that Rev1:1 / 1:19c / 4:1 also covers the Subject of...) ]