The theme of Paul's letter to Philemon is forgiveness. In this letter, Paul appeals to Philemon to forgive his runaway slave, Onesimus, and accept him back as a brother in Christ. Paul even offers to pay any debts that Onesimus owed Philemon. The letter emphasizes the importance of forgiveness, reconciliation, and love among Christians, and it also shows how Paul used his authority to advocate for Onesimus and promote Christian unity.