Philemon is a letter with an interesting back story-
Paul is sending a runaway slave or servant back to his old master, carrying his letter. The master? Philemon, and a believer. The slave? Onesimus, another believer.

Paul challenges Philemon's possible responses to the return of his slave by appealing to our submission to Christ, the gift of grace we have received in him, and the gift of life given by Paul (through the sharing of the gospel) to Philemon.