Paul's Epistle to Philemon is the shortest of the Pauline epistles (only 335 words). Hell, I've written postcards longer than that. But two things about it seem worth mentioning. First, it is believed to be genuinely Pauline, unlike some of the letters, which are by someone posing as Paul. Second, Paul urges forgiveness of Onesimus, a slave who stole something from Philemon and ran away. Paul urges Philemon to forgive Onesimus and welcome him back into the household as a brother in Christ.
It's a good thing Philemon did this because he and his wife Apphia were subsequently stoned to death for being Christians. While martyrdom might be a frequent ticket to recognition of sainthood, it's always better to show up for judgment with a few forgivenesses in your account.