And now ye know what withholdeth that he might be revealed in his time.
Now ye know - by my having told you. The power was one 'known' to the Thessalonians.
What withholdeth - holds him back. 'The power that х to (G3588) katechon (G2722)] keeps in check' the man of sin from his development is the moral and conservative influence of states (Olshausen): human polity as a coercive power: "he who now letteth" refers to those who rule that polity by which the upbursting of godlessness is kept down (Alford). Legality, as opposed to 'lawlessness.' The "what withholdeth" is the general hindrance; "he who now letteth," the person in whom that hindrance is summed up. Romanism, as a forerunner, was kept in check by the Roman emperor (the then representative of the coercive power), until Constantine, having removed the seat of empire to Constantinople, the Roman Bishop first raised himself to precedency, then to primacy, then to sole empire above the secular power. The historical fact from which Paul starts was probably the emperor Claudius' expulsion of the Jews, the anti-Christian adversary in Paul's day, from Rome, thus 'withholding' them in their attacks on Christianity. This suggested the principle about to find its final fulfillment in the removal of the withholding person or authority, whereupon Antichrist in his worst shape shall arise.
That he might be. Ye know what keeps him back, in God's purposes, from being sooner manifested-`in order that he may be revealed in his own (Antichrist's proper) time' appointed by God (cf. Daniel 11:35). This will be when the civil polity derived from the Roman empire, which will be, in its last form, divided into ten kingdoms (Revelation 17:3; Revelation 17:11-13), shall, with its representative head ("he who now letteth" х ho (G3588) katechoon (G2722)]: "withholdeth," as in 2 Thessalonians 2:6), yield to the prevalent 'lawlessness' of which 'the lawless one' is the embodiment. The elect Church and the Spirit, De Burgh suggests, are the withholding power; but they, to 'the end of the age,' shall never be wholly "taken out of the way" (Matthew 28:20). However, as the testimony of the elect Church, and the Spirit in her, hold back now the apostasy, it is possible that, though a few shall be faithful even then, yet the full energy of the Spirit in the visible Church, counteracting the 'working' [energy: energeitai] of 'the mystery of lawlessness' by the elect's testimony, shall be so far 'taken out of the way' as to admit the manifestation of 'the lawless one' (Revelation 11:3-12). The rapture of the elect Church to meet the Lord in the air (Isaiah 26:20; 1 Thessalonians 4:17) BEFORE Antichrist's brief reign and the great Jewish tribulation, clears away some difficulty. The Thessalonians might easily "know" this through Paul's instruction.