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.
Ver. 3, 4. "Let no man beguile you in any wise: for it will not be, except the falling away come first, and the man of sin be revealed, the son of perdition, he that opposes and exalts himself against all that is called God or that is worshipped; so that he sits in the temple of God, setting himself forth as God."- Chrysostom, Homilies on 2 Thessalonians, Homily 3, at 2 Thessalonians 2:3-4
Here he discourses concerning the Antichrist, and reveals great mysteries. What is "the falling away?" He calls him Apostasy, as being about to destroy many, and make them fall away. So that if it were possible, He says, the very Elect should be offended. From Matthew 24:24 And he calls him "the man of sin." For he shall do numberless mischiefs, and shall cause others to do them. But he calls him "the son of perdition," because he is also to be destroyed. But who is he? Is it then Satan? By no means; but some man, that admits his fully working in him. For he is a man. "And exalts himself against all that is called God or is worshipped." For he will not introduce idolatry, but will be a kind of opponent to God; he will abolish all the gods, and will order men to worship him instead of God, and he will be seated in the temple of God, not that in Jerusalem only, but also in every Church. "Setting himself forth," he says; he does not say, saying it, but endeavoring to show it. For he will perform great works, and will show wonderful signs.
Let us not follow the opinion of some commentators and suppose him to be either the Devil or some demon, but rather, one of the human race, in whom Satan will wholly take up his residence in bodily form. ". . .and a mouth uttering overweening boasts..." (cf. II Thess. 2). For this is the man of sin, the son of perdition, and that too to such a degree that he dares to sit in the temple of God, making himself out to be like God.- Jerome, Commentary on Daniel, at Daniel 7:8
Cyril of Jerusalem:
And what comes to pass after this? He says next, When therefore you see the abomination of desolation, which was spoken of by Daniel the Prophet, standing in the Holy Place, let him that reads understand. And again, Then if any man shall say unto you, Lo, here is the Christ, or, Lo, there; believe it not. Hatred of the brethren makes room next for Antichrist; for the devil prepares beforehand the divisions among the people, that he who is to come may be acceptable to them. But God forbid that any of Christ's servants here, or elsewhere, should run over to the enemy! Writing concerning this matter, the Apostle Paul gave a manifest sign, saying, For that day shall not come, except there came first the falling away, and the man of sin be revealed, the son of perdition, who opposes and exalts himself against all that is called God, or that is worshiped; so that he sits in the temple of God, showing himself that he is God. Do you not remember that when I was yet with you, I told you these things? And now you know that which restrains, to the end that he may be revealed in his own season. For the mystery of iniquity does already work, only there is one that restrains now, until he be taken out of the way. And then shall the lawless one be revealed, whom the Lord Jesus shall slay with the breath of His mouth, and shall destroy with the brightness of His coming. Even him, whose coming is after the working of Satan, with all power and signs and lying wonders, and with all deceit of unrighteousness for them that are perishing. [2 Thessalonians 2:3-10] Thus wrote Paul, and now is the falling away. For men have fallen away from the right faith; and some preach the identity of the Son with the Father, and others dare to say that Christ was brought into being out of nothing. And formerly the heretics were manifest; but now the Church is filled with heretics in disguise. For men have fallen away from the truth, and have itching ears. [2 Timothy 4:3] Is it a plausible discourse? All listen to it gladly. Is it a word of correction? All turn away from it. Most have departed from right words, and rather choose the evil, than desire the good. This therefore is the falling away, and the enemy is soon to be looked for: and meanwhile he has in part begun to send forth his own forerunners, that he may then come prepared upon the prey. Look therefore to yourself, O man, and make safe your soul. The Church now charges you before the Living God; she declares to you the things concerning Antichrist before they arrive. Whether they will happen in your time we know not, or whether they will happen after you we know not; but it is well that, knowing these things, you should make yourself secure beforehand.- Catechetical Lectures, Lecture 15, Section 9