John Calvin Commentary 2 Thessalonians 2:5

"Remember ye not, that, when I was yet with you, I told you these things?" — 2 Thessalonians 2:5 (ASV)

Do you not remember? This added considerable weight to the doctrine, because they had previously heard it from Paul's own mouth, so they might not think he had contrived it at that moment. And since he had given them early warning about the reign of Antichrist and the devastation that was coming upon the Church—when no question had yet been raised about such things—he saw beyond all doubt that the doctrine was especially useful to know.

And, unquestionably, this is indeed so. Those whom he addressed were destined to see many things that would trouble them. And when future generations would see a large proportion of those who had professed the faith of Christ revolt from piety—maddened, as it were, by a gadfly, or rather by a fury—what could they do but waver?

This, however, was like a brazen wall—that matters were so appointed by God, because human ingratitude deserved such vengeance. Here we can see how forgetful people are in matters affecting their everlasting salvation. We must also observe Paul’s mildness. For while he could have been vehemently incensed, he only mildly reproves them. For it is a fatherly way of rebuking them to say that they had allowed forgetfulness of such an important and useful matter to steal in upon their minds.