Jamieson, Fausset and Brown on 2 Thessalonians 2:11

11. for this cause--because "they received not the love of the truth." The best safeguard against error is "the love of the truth."

shall send--Greek, "sends," or "is sending"; the "delusion" is already beginning. God judicially sends hardness of heart on those who have rejected the truth, and gives them up in righteous judgment to Satan's delusions (Is 6:9, 10; Ro 1:24-26, 28). They first cast off the love of the truth, then God gives them up to Satan's delusions, then they settle down into "believing the lie": an awful climax (1Ki 22:22, 23; Eze 14:9; Job 12:16; Mt 24:5, 11; 1Ti 4:1).

strong delusion--Greek, "the powerful working of error," answering to the energizing "working of Satan" (2Th 2:9); the same expression as is applied to the Holy Ghost's operation in believers: "powerful" or "effectual (energizing) working" (Ep 1:19).

believe a lie--rather, "the lie" which Antichrist tells them, appealing to his miracles as proofs of it ... (2Th 2:9).