2 Thessalonians 2:7

The mystery of iniquity already worketh, [8] or is now wrought, by the precursors of antichrist; i.e. by infidels and heretics. For, as St. John says, there are many antichrists, precursors to the great antichrist, and enemies of Christ. (1 John chap. ii.) (Witham) --- That he who now holdeth, do hold. That is, let each one remain in the faith which he has received of us, and let him not permit himself to be deceived by any discourse, as coming from us. Or rather, let those who shall then be in the world keep their faith, remaining firm in their belief and attachment to the Church of Christ, until antichrist, that man of iniquity, shall be taken away. (Calmet) --- According to others, it is an admonition to the faithful not to be beguiled during this day of trial by such, as under the garb of religion, and with an ostentatious parade of zeal for the holy Scriptures, seek to deceive them. When the mystery of sin shall be revealed, then shall we plainly discover that apostacy from the Catholic Church is the obvious and certain road to a dereliction of all religious principle; yes, to antichristianism and to atheism. (Haydock)

[BIBLIOGRAPHY]

Mysterium jam operatur iniquitatis, Greek: to musteroin ede energeitai.