9 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 worshipped; 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</em>. 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. 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.
Source: Catechetical Lectures (New Advent)