One of Paul's shortest and the most explosive letters, the book of Philemon demonstrates the gospel through action. It's written to Philemon whose slave Onesimus ran away and became a believer under Paul's teaching.
Paul urges Philemon to forgive Onesimus and accept him back as an equal. Because they are both believers, God's grace and healing mercy have made them partners under the new humanity Jesus' established.
While Paul doesn't mention Jesus' death and resurrection directly in this book, he offers to take on any punishment Onesimus deserves, a demonstration of what Jesus did on the cross. He encourages Philemon to do the same as he reconciles to Onesimus before God.