Slave turned brother

Paul wants Philemon to treat his returning slave with kindness and love. Roman law allowed slaveowners to punish escaped slaves harshly. Onesimus may have run away because of a disagreement with Philemon. In this kind of situation, a slave might ask a trusted third party (like Paul) to intervene. What does this letter say about how Christians should treat one another? What can we learn from Paul’s example and the actions he took on behalf of Onesimus?