Augustine of Hippo

Then as for the words, "And now ye know what withholdeth," i.e., ye know what hindrance or cause of delay there is, "that he might be revealed in his own time;" they show that he was unwilling to make an explicit statement, because he said that they knew. And thus we who have not their knowledge wish and are not able even with pains to understand what the apostle referred to, especially as his meaning is made still more obscure by what he adds. For what does he mean by "For the mystery of iniquity doth already work: only he who now holdeth, let him hold until he be taken out of the way: and then shall the wicked be revealed?" I frankly confess I do not know what he means. I will nevertheless mention such conjectures as I have heard or read.

Some think that the Apostle Paul referred to the Roman empire, and that he was unwilling to use language more explicit, lest he should incur the calumnious charge of wishing ill to the empire which it was hoped would be eternal; so that in saying, "For the mystery of iniquity doth already work," he alluded to Nero, whose deeds already seemed to be as the deeds of Antichrist. And hence some suppose that he shall rise again and be Antichrist. Others, again, suppose that he is not even dead, but that he was concealed that he might be supposed to have been killed, and that he now lives in concealment in the vigor of that same age which he had reached when he was believed to have perished, and will live until he is revealed in his own time and restored to his kingdom. But I wonder that men can be so audacious in their conjectures. However, it is not absurd to believe that these words of the apostle, "Only he who now holdeth, let him hold until he be taken out of the way," refer to the Roman empire, as if it were said, "Only he who now reigneth, let him reign until he be taken out of the way." "And then shall the wicked be revealed:" no one doubts that this means Antichrist. But others think that the words, "Ye know what withholdeth," and "The mystery of iniquity worketh," refer only to the wicked and the hypocrites who are in the Church, until they reach a number so great as to furnish Antichrist with a great people, and that this is the mystery of iniquity, because it seems hidden...