From Matthew Henry's Commentary on this passage (Julius you should like this):
"In these words the apostle confutes the error against which he had cautioned them, and gives the reasons why they should not expect the coming of Christ as just at hand. There were several events previous to the second coming of Christ; in particular, he tells them there would be,
I. A general apostasy, there would come a falling away first, v. 3. By this apostasy we are not to understand a defection in the state, or from civil government, but in spiritual or religious matters, from sound doctrine, instituted worship and church government, and a holy life. The apostle speaks of some very great apostasy, not only of some converted Jews or Gentiles, but such as should be very general, though gradual, and should give occasion to the revelation of rise of antichrist, that man of sin. This, he says (v. 5), he had told them of when he was with them, with design, no doubt, that they should not take offence nor be stumbled at it. And let us observe that no sooner was Christianity planted and rooted in the world than there began to be a defection in the Christian church. It was so in the Old-Testament church; presently after any considerable advance made in religion there followed a defection: soon after the promise there was revolting; for example, soon after men began to call upon the name of the Lord all flesh corrupted their way,-soon after the covenant with Noah the Babel-builders bade defiance to heaven,-soon after the covenant with Abraham his seed degenerated in Egypt,-soon after the Israelites were planted in Canaan, when the first generation was worn off, they forsook God and served Baal,-soon after God's covenant with David his seed revolted, and served other gods,-soon after the return out of captivity there was a general decay of piety, as appears by the story of Ezra and Nehemiah; and therefore it was no strange thing that after the planting of Christianity there should come a falling away.
II. A revelation of that man of sin, that is (v. 3), antichrist would take his rise from this general apostasy. The apostle afterwards speaks of the revelation of that wicked one (v. 8), intimating the discovery which should be made of his wickedness, in order to his ruin: here he seems to speak of his rise, which should be occasioned by the general apostasy he had mentioned, and to intimate that all sorts of false doctrines and corruptions should centre in him. Great disputes have been as to who or what is intended by this man of sin and son of perdition: and, if it be not certain that the papal power and tyranny are principally or only intended, yet this is plain, What is here said does very exactly agree thereto. For observe,
1. The names of this person, or rather the state and power here spoken of. He is called the man of sin, to denote his egregious wickedness; not only is he addicted to, and practises, wickedness himself, but he also promotes, countenances, and commands sin and wickedness in others; and he is the son of perdition, because he himself is devoted to certain destruction, and is the instrument of destroying many others both in soul and body. These names may properly be applied, for these reasons, to the papal state; and thereto agree also,
2. The characters here given, v. 4.
(1.) That he opposes and exalts himself above all that is called God, or is worshipped; and thus have the bishops of Rome not only opposed God's authority, and that of the civil magistrates, who are called gods, but have exalted themselves above God and earthly governors, in demanding greater regard to their commands than to the commands of God or the magistrate.
(2.) As God, he sits in the temple of God, showing himself that he is God. As God was in the temple of old, and worshipped there, and is in and with his church now, so the antichrist here mentioned is some usurper of God's authority in the Christian church, who claims divine honours; and to whom can this better apply than to the bishops of Rome, to whom the most blasphemous titles have been given, as Dominus Deus noster papa-Our Lord God the pope; Deus alter in terrâ-Another God on earth; Idem est dominium Dei et papae-The dominion of God and the pope is the same?
3. His rise is mentioned, v. 6, 7. Concerning this we are to observe two things:-
(1.) There was something that hindered or withheld, or let, until it was taken away. This is supposed to be the power of the Roman empire, which the apostle did not think fit to mention more plainly at that time; and it is notorious that, while this power continued, it prevented the advances of the bishops of Rome to that height of tyranny to which soon afterwards they arrived.
(2.) This mystery of iniquity was gradually to arrive at its height; and so it was in effect that the universal corruption of doctrine and worship in the Romish church came in by degrees, and the usurpation of the bishops of Rome was gradual, not all at once; and thus the mystery of iniquity did the more easily, and almost insensibly, prevail. The apostle justly calls it a mystery of iniquity, because wicked designs and actions were concealed under false shows and pretences, at least they were concealed from the common view and observation. By pretended devotion, superstition and idolatry were advanced; and, by a pretended zeal for God and his glory, bigotry and persecution were promoted. And he tells us that this mystery of iniquity did even then begin, or did already work. While the apostles were yet living, the enemy came, and sowed tares; there were then the deeds of the Nicolaitans, persons who pretended zeal for Christ, but really opposed him. Pride, ambition, and worldly interest of church-pastors and church-rulers, as in Diotrephes and others, were the early working of the mystery of iniquity, which, by degrees, came to that prodigious height which has been visible in the church of Rome.
4. The fall or ruin of the antichristian state is declared, v. 8. The head of this antichristian kingdom is called that wicked one, or that lawless person who sets up a human power in competition with, and contradiction to, the divine dominion and power of the Lord Jesus Christ; but, as he would thus manifest himself to be the man of sin, so the revelation or discovery of this to the world would be the sure presage and the means of his ruin. The apostle assures the Thessalonians that the Lord would consume and destroy him; the consuming of him precedes his final destruction, and that is by the Spirit of his mouth, by his word of command; the pure word of God, accompanied with the Spirit of God, will discover this mystery of iniquity, and make the power of antichrist to consume and waste away; and in due time it will be totally and finally destroyed, and this will be by the brightness of Christ's coming. Note, The coming of Christ to destroy the wicked will be with peculiar glory and eminent lustre and brightness.
5. The apostle further describes the reign and rule of this man of sin. Here we are to observe,
(1.) The manner of his coming, or ruling, and working: in general, that it is after the example of Satan, the grand enemy of souls, the great adversary of God and man. He is the great patron of error and lies, the sworn enemy of the truth as it is in Jesus and all the faithful followers of Jesus. More particularly, it is with Satanical power and deceit. A divine power is pretended for the support of this kingdom, but it is only after the working of Satan. Signs and wonders, visions and miracles, are pretended; by these the papal kingdom was first set up, and has all along been kept up, but they have false signs to support false doctrines; and lying wonders, or only pretended miracles that have served their cause, things false in fact, or fraudulently managed, to impose upon the people: and the diabolical deceits with which the antichristian state has been supported are notorious. The apostle calls it all deceivableness of unrighteousness, v. 10. Others may call them pious frauds, but the apostle called them unrighteous and wicked frauds; and, indeed, all fraud (which is contrary to truth) is an impious thing. Many are the subtle artifices the man of sin has used, and various are the plausible pretences by which he had beguiled unwary and unstable souls to embrace false doctrines, and submit to his usurped dominion.
(2.) The persons are described who are his willing subjects, or most likely to become such, v. 10. They are such as love not the truth that they may be saved. They heard the truth (it may be), but they did not love it; they could not bear sound doctrine, and therefore easily imbibed false doctrines; they had some notional knowledge of what was true, but they indulged some powerful prejudices, and so became a prey to seducers. Had they loved the truth, they would have persevered in it, and been preserved by it; but no wonder if they easily parted with what they never had any love to. And of these persons it is said that they perish or are lost; they are in a lost condition, and in danger to be lost for ever. For,
6. We have the sin and ruin of the subjects of antichrist's kingdom declared, v. 11, 12.
(1.) Their sin is this: They believed not the truth, but had pleasure in unrighteousness: they did not love the truth, and therefore they did not believe it; and, because they did not believe the truth, therefore they had pleasure in unrighteousness, or in wicked actions, and were pleased with false notions. Note, An erroneous mind and vicious life often go together and help forward one another.
(2.) Their ruin is thus expressed: God shall send them strong delusions, to believe a lie. Thus he will punish men for their unbelief, and for their dislike of the truth and love to sin and wickedness; not that God is the author of sin, but in righteousness he sometimes withdraws his grace from such sinners as are here mentioned; he gives them over to Satan, or leaves them to be deluded by his instruments; he gives them up to their own hearts' lusts, and leaves them to themselves, and then sin will follow of course, yea, the worst of wickedness, that shall end at last in eternal damnation. God is just when he inflicts spiritual judgments here, and eternal punishments hereafter, upon those who have no love to the truths of the gospel, who will not believe them, nor live suitably to them, but indulge false doctrines in their minds, and wicked practices in their lives and conversations.