We have left behind the epistles of Paul, meaning we are no longer discussing letters written to their namesakes, like Timothy, Titus and Philemon, but are back to talking about letters written by their namesakes.  Therefore, as you surely have guessed, Peter wrote the books with his name on them, John will do the same to his, and so will Jude.  You already know about James also doing this by now, I imagine.  With the author identified, why does Peter say we should be holy?

-Because God said so-

-Because it is the will of God-

-Because it is sin to not be-

-Because God is holy-