2:7

the mystery of this lawlessness: For “mystery,” Paul uses the Greek word my·steʹri·on, which describes something secret and beyond normal understanding. It is used in a similar sense at Re 17:5, 7. (For a discussion of other occurrences of this Greek word, see study note on Mt 13:11.) In the case of “the man of lawlessness,” there was an element of mystery because the leadership among apostates had not yet established itself as an identifiable group. But that mystery was already at work because apostates were infiltrating the congregation, acting as a subversive influence toward lawlessness.​—Ac 15:24; see study note on 2Th 2:3.

the one who is right now acting as a restraint: Paul here repeats the Greek word for “acting as a restraint” from the preceding verse, adding the time element “right now.” He likely refers to the restraining influence of the apostles. (See study note on 2Th 2:6.) Decades later, about 98 C.E., the apostle John suggested that it was then “the last hour” of the apostolic age and that apostasy was spreading. (1Jo 2:18) That last restraint against apostasy was “out of the way” after John died about 100 C.E.