John Calvin Commentary 2 Peter 3:1

"This is now, beloved, the second epistle that I write unto you; and in both of them I stir up your sincere mind by putting you in remembrance;" — 2 Peter 3:1 (ASV)

So that they would not be wearied with the Second Epistle as if the first was sufficient, he says that it was not written in vain, because they needed to be often stirred up. To make this more evident, he shows that they could not be beyond danger unless they were well fortified, because they would have to contend with desperate men, who would not only corrupt the purity of the faith by false opinions, but also do what they could to completely subvert the faith.

By saying, I stir up your pure mind, he means the same as if he had said, “I wish to awaken you to a sincerity of mind.” And the words ought to be explained this way: “I stir up your mind so that it may be pure and bright.” For the meaning is, that the minds of the godly become dim and, as it were, contract rust when admonitions cease. But from this we also learn that even learned men become, in a way, drowsy, unless they are stirred up by constant warnings.

It now appears what the use of admonitions is, and how necessary they are; for the sloth of the flesh smothers the truth once received and renders it inefficient, unless the goads of warnings come to its aid. It is not enough, then, for men to be taught what they ought to be, but godly teachers are needed to do this second part: to deeply impress the truth on the memory of their hearers. And since men are, by nature, for the most part, fond of novelty and therefore inclined to be fastidious, it is useful for us to bear in mind what Peter says, so that we may not only willingly allow ourselves to be admonished by others, but that everyone may also practice continually recalling the truth, so that our minds may become resplendent with its pure and clear knowledge.