Let no one deceive you in any way; for that day will not come unless the rebellion comes first and the lawless one is revealed, the one destined for destruction. He opposes and exalts himself above every so-called god or object of worship, so that he takes his seat in the temple of God, declaring himself to be God[…] The coming of the lawless one is apparent in the working of Satan, who uses all power, signs, lying wonders, and every kind of wicked deception for those who are perishing, because they refused to love the truth and so be saved. For this reason God sends them a powerful delusion, leading them to believe what is false, so that all who have not believed the truth but took pleasure in unrighteousness will be condemned. (2 Thessalonians 2:3-4, 9-12, NRSV)
Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition; Who opposeth and exalteth himself above all that is called God, or that is worshipped; so that he as God sitteth in the temple of God, shewing himself that he is God[…] Even him, whose coming is after the working of Satan with all power and signs and lying wonders, And with all deceivableness of unrighteousness in them that perish; because they received not the love of the truth, that they might be saved. And for this cause God shall send them strong delusion, that they should believe a lie: That they all might be damned who believed not the truth, but had pleasure in unrighteousness. (2 Thessalonians 2:3-4, 9-12, KJV)
Ooo, the Man of Sin. This guy is bad news. He raises himself up above God (a big no-no) and is pretty much in league with Satan. Doesn't get more terrible than that. Even God gets in on a little evil action here. Those who don't see his wickedness get bewitched by God into thinking the Man of Sin is great. So which came first? The unbelief or the God that caused this unbelief?