John Chrysostom

"And now ye know that which restraineth, to the end that he may be revealed in his own season. For the mystery of lawlessness doth already work: only there is one that restraineth now, until he be taken out of the way." One may naturally enquire, what is that which withholdeth, and after that would know, why Paul expresses it so obscurely. What then is it that withholdeth, that is, hindereth him from being revealed? Some indeed say, the grace of the Spirit, but others the Roman empire, to whom I most of all accede. Wherefore? …

Homily on 2 Thessalonians 4 ↗

Theophylact of Ohrid

That which "restrains" means an obstacle — that which hinders. What is this? Some assert that it is the grace of the Holy Spirit; others — the Roman state, and with the latter Saint John Chrysostom also agrees. For as long as it is not destroyed, the Antichrist will not have the ability to do as he pleases. Therefore Paul also spoke covertly: he did not wish to bring upon himself needless enmity and useless danger. …

Commentary on 2 Thessalonians ↗

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. …

The City of God, Book 20 ↗

Thomas Aquinas

Above, the Apostle foretold the coming and the wrongdoing of the Antichrist; here he explains the reason for its delay. And first, he shows that they know this cause; second, he sets out this cause in an obscure manner, at "for the mystery." He says, therefore: I say that the man of sin must be revealed. "And now you know what withholds", i.e., what is the cause of his delay. …

Commentary on 2 Thessalonians ↗