'Letters' in the Bible

Dear friends, this is now the second of two letters I am writing to you, in which I have been trying to stimulate your pure minds by reminding you

And be certain that the long waiting of the Lord is for salvation; even as our brother Paul has said in his letters to you, from the wisdom which was given to him;

He speaks about these things in all his letters in which there are some matters that are hard to understand. The untaught and unstable twist them to their own destruction, as they also do with the rest of the Scriptures.