Philemon is the shortest book of the New Testament. Philemon was a slave owner and a believer from Colossae. One of his slaves, Onesimus, had wronged him (perhaps stolen from him) and then run away. In the ancient world, such an act was punishable by death. During the time after his escape, Onesimus became a Christian under Paul’s ministry. Paul writes Philemon to convince him to allow Onesimus to return without penalty, and as a full brother in Christ.
Paul’s rhetorical gifts are in full flourish in this brief letter. By referring to himself as a prisoner in verse 1, Paul is signaling to Philemon that he should see Paul and Onesimus in a similar light—being harsh to the Onesimus would mean being harsh to Paul. Paul also stresses the Christian bond that the three of them have, and for this reason Onesimus is a brother first and slave second.
With less subtlety, Paul continues by offering to pay back to Philemon what Onesimus owes him, even though Paul says Philemon owes Paul his whole life. Finally, Paul tells Philemon to prepare a guest room for him, as he intends to pay a visit as soon as he is released. Paul signals respectfully that he will be checking up on Philemon at the first opportunity.
– Peter Enns