For me, the delusion is the lying spirit (breath) which says “peace and safety” as found in 1 Thessalonians 5:3:
1 Thessalonians 5:3 KJV
(3) For when they shall say, Peace and safety; then sudden destruction cometh upon them, as travail upon a woman with child; and they shall not escape.
This reminds me of the lying spirit (breath) in the mouths of the 400 false prophets who advised Ahab and with their persuasion led to his death. Ahab was persuaded by their unified message (the lying spirit) to think that he should go into battle with the Aramaeans/Syrians and be victorious/prosperous and return in peace.
1 Kings 22:27-28 KJV
(27) And say, Thus saith the king, Put this fellow in the prison, and feed him with bread of affliction and with water of affliction, until I come in peace.
Micaiah the lone true prophet of God: if you return in peace at all, then the LORD has not spoken by me.
(28) And Micaiah said, If thou return at all in peace, the LORD hath not spoken by me. And he said, Hearken, O people, every one of you.
As we know, Ahab preferred to listen to the false prophets (the lying spirit put in their mouths saying peace and safety so to speak) over the lone true prophet and died.
The strong delusion (the lying breath) is one saying “peace and safety” when there is none. They think that they have “peace” and believe THE lie rather than "a" new lie. The lie that is to be practiced is the same lie of the serpent from the very beginning.
And with all deceivableness of unrighteousness in them that perish; because they received not the love of the truth, that they might be saved. 11 And for this cause God shall send them strong delusion, that they should believe a lie: 12 That they all might be damned who believed not the truth, but had pleasure in unrighteousness. And for this cause God shall send them strong delusion, that they should believe a lie: 2 Thessalonians 2:10-12 KJV
The lie: “you shall not surely die” (a peace and safety message). It should be translated “dying, you shall not die” or “killing, you shall not die”. One “kills” with the tongue when it is used to speak evil of another judging them as evildoers (judge, jury and executioner of another).
“For God does know….” is saying that one is a judge for God supposedly and judging others is doing His will.
“Your eyes shall be opened…” is saying that you shall be seen as wise.
“You shall be as gods knowing good and evil”….is saying you shall be as JUDGES (Elohim= gods, judges). Judges know who or what is good and evil.
Genesis 3:4-5 KJV
(4) And the serpent said unto the woman, Ye shall not surely die:
(5) For God doth know that in the day ye eat thereof, then your eyes shall be opened, and ye shall be as gods, knowing good and evil.
If one speaks evil of another, he JUDGES his brother and is not a doer of the law but a judge (the lie: you shall be as gods or judges knowing good and evil).
James 4:11-12 KJV
(11) Speak not evil one of another, brethren. He that speaketh evil of his brother, and judgeth his brother, speaketh evil of the law, and judgeth the law: but if thou judge the law, thou art not a doer of the law, but a judge.
Some label others as “evildoers” using the commandments and doctrines of men (which determine who is good and evil) when there is only One Lawgiver who is able to save and to destroy.
(12) There is one lawgiver, who is able to save and to destroy: who art thou that judgest another?
Thanks for reading and considering.