John Calvin Commentary 2 Peter 3:15

"And account that the longsuffering of our Lord is salvation; even as our beloved brother Paul also, according to the wisdom given to him, wrote unto you;" — 2 Peter 3:15 (ASV)

The long-suffering of our Lord. He takes it for granted that Christ defers the day of his coming because he has a regard for our salvation. He therefore encourages the faithful, because in a longer delay they have evidence of their own salvation. Thus, what usually discourages others through weariness, he wisely turns to the opposite effect.

Even as our beloved brother Paul. We may easily gather from the Epistle to the Galatians, as well as from other places, that unprincipled men, who went about everywhere to disturb the churches, in order to discredit Paul, used the pretense that he did not agree well with the other Apostles. It is therefore probable that Peter referred to Paul in order to show their agreement; for it was very necessary to remove the basis for such slander.

And yet, when I examine all things more closely, it seems to me more probable that this Epistle was composed by another according to what Peter communicated, than that it was written by himself, for Peter himself would have never spoken in this way. But it is enough for me that we have a witness of his doctrine and of his goodwill, who presented nothing contrary to what he himself would have said.