This wonderful yet short book is often one of the most misunderstood books of the Bible. The common thought is that Onesimus is a runaway slave who comes into contact with Paul. While with Paul, Onesimus commits his life to Christ and begins to grow in the faith. Once it is time for Onesimus to go back to his "owner" Philemon, Paul sends the letter to encourage Philemon to let Onesismus be free. This understanding is adequate, however it contains a major assumption which is hard to support in the text. No where does it tell us Onesimus is a runaway slave. It is entirely possible he is a slave, but the runaway part in conjecture.
It is entirely possible Onesimus was sent by Philemon to care for Paul in Rome for a defined amount of time. The letter is written as that time frame has come to a close. While with Paul Onesimus becomes a follower of the Way. Paul recognizing the condition of Onesimus, who most likely owes Philemon money, that is why he is a slave, tells Philemon he will settle the account.
If we look carefully we can see ourselves in the letter. God created us and send us into life. We owe God a great debt, due to sin. While on this earthly voyage we encounter Christ. If we choose to follow Christ, a letter is then sent on our behalf so to speak. Jesus tells the Father that he will settle the debt that we might be free. It is not that we were runaway slaves, it is that we have not been as useful to God because of our time away. Through Jesus we are made useful for the work of God. Interesting that Onesimus means useful.
Can you see yourself in the letter?