The letters of Paul keep getting shorter, and in fact, that is probably why they were arranged in the order they are – from longest to shortest. Philemon is a friend of Paul’s, and Paul writes him because he found one of Philemon’s slaves, a man named Onesimus. The point of the letter is for Paul to convince Philemon to accept Onesimus back without punishing him, and even more, to treat him as a brother and not as a slave.