Paul's letter to Philemon was written to support Onesimus as he returned to his master. In the letter, Paul asked Philemon to receive Onesimus as a brother in Christ and forgive him for his past actions. Onesimus was a slave who had run away from Philemon, and through his journey, he met Paul, who introduced him to Christianity. Paul then wrote the letter to Philemon on behalf of Onesimus, urging him to accept him back as a brother in Christ and not punish him for running away.