Paul was in prison when he wrote his letter to Philemon, the Christian master of the runaway slave Onesimus. Yet in his opening words he identifies himself, not as a prisoner of Rome, but “a prisoner of Christ Jesus” (Phlm 1). The way Paul saw it, he was in prison by the will of God. And while others may have been slaves of their earthly masters, he was God’s slave, God’s servant (see, e.g., Titus 1:1: “Paul, a slave of God”). For faith changes everything.
Faith also changed everything in the life of Paul’s new protégé, Onesimus. Apparently, Onesimus, a slave, had run away from Philemon, his master, and met Paul in prison in Rome. There, it appears, Paul shared the gospel with this desperate man, and he was converted to Christ. Accordingly, Paul refers to Onesimus as “my child, whom I fathered while in chains” (Phlm 10). Onesimus, whose name means “useful,” would have been “useful” for Paul, but he chose to send him back to his master, so that he would be “useful”—truly useful, now that he had become a Christian—to him.
In his letter to Philemon, Paul appealed to his friend to receive Onesimus back “no longer as a slave, but more than a slave—as a dearly loved brother” (Phlm 16). Onesimus was a changed man, and Paul urged Philemon to recognize that his relationship with Onesimus had changed as well: they were now brothers in Christ. This is a wonderful example of how Paul envisioned Christianity to transform, not only individuals, but also social structures such as slavery.
At the conclusion of his letter, Paul has one more personal request which he saved for last: “But meanwhile, also prepare a guest room for me, for I hope that through your prayers I will be restored to you” (Phlm 22). At the time of writing, Paul was still in prison, but he was hopeful that he would be released soon, so hopeful, in fact, that he already began to make preparations to visit Philemon and Onesimus—including reserving a guest room! That’s faith.
P.S.: Note that Paul is not alone when he writes this letter. Actually, half of the Gospel writers are with him at the time of writing: both Mark and Luke (Phlm 24)! This shows how close-knit the Christian brotherhood was in the early church.