Question: What might be included in “the strong delusion” in 2 Thessalonians:2:11? And is it already here, or does it only take effect after the Rapture?
Answer: Paul says that the “delusion” specifically causes those who “received not the love of the truth” to be deceived by the “lying wonders” performed by Antichrist (“that Wicked...whose coming is after the working of Satan”) and by his “deceivableness of unrighteousness...” (2 Thes:2:8-10). Clearly, this applies to those left behind at the Rapture in a world ruled by Antichrist; but it would seem that their rejection of the truth preceded and prepared them for “that man of sin” (v. 3,4) and thus could occur, at least to a lesser degree, before the Rapture.
The delusion causes “that they should believe a lie: that they all might be damned” (v. 11,12). Revelation:13:2,4,8 say the world will worship both “the beast” (Antichrist) and “the dragon [Satan who] gave him his power, and his seat, and great authority.” Thus we could conclude that the lie is Antichrist’s claim that he is God: “he as God sitteth in the temple of God, shewing himself that he is God” (2 Thes:2:4). The lie would seem to be the total delusion under Antichrist and the false prophet, which would include taking his mark and bowing down to his image. All who do so are damned (Rv 14:9-11).