While in prison, Paul writes this letter to another Christian, Philemon. Paul has met with and converted a man named Onesimus, probably a runaway slave, and he writes to Philemon, Onesimus's legal owner, to tell him that he is sending him back. Appealing to Philemon's personal debt to him, Paul urges him to welcome Onesimus as though he were Paul himself, and to accept him as a fellow brother in Christ.
(Summary by Leon Mire)