I would have been glad to keep him with me, in order that he might serve me on your behalf during my imprisonment for the gospel, but I preferred to do nothing without your consent in order that your goodness might not be by compulsion but of your own accord.
In Paul's short postcard epistle, we have the story of Philemon and Onesimus told to us as a brash reminder of the transforming power of the gospel to turn human imposed cultural norms upside down. Philemon owned a slave, Onesimus. Philemon was a Christian, who evidently had been directly impacted by Paul's ministry and had served the advance of the gospel for the apostle in the past.
Onesimus robbed his master one day and fled to Rome. There, in God's sovereign graces, he came into contact with Paul and the gospel and became a Christian. Paul sent him back to his master with this letter, and in this real life story we see the reconciling power of the gospel as Paul makes his confident appeal for Philemon to accept the repentant former thief back into his household and to love him now as a brother in Christ.
Philemon has every right under Roman law to execute Onesimus and make an example of the thieving trust-breaker. He was not obligated to make him a servant again. Culturally it made no sense to trust Onesimus again. But the gospel rebirths us into new creations in Christ. That is the appeal Paul makes for Onesimus. It is because Onesimus believed the gospel and was now living for the gospel that Paul could make this outrageous demand that the transformation be complete, confident that Philemon would feel the compelling force of the love of God in Christ in the gospel to forgive the sinner and take him into his home and his heart now as his brother.
Paul could have appealed to apostolic command to pressure Philemon into such actions. But instead, his source of argument is the simple power of the gospel. Paul trusts that Philemon will love his brother in Christ because that is what Christians who believe the gospel will do. He believes that the good things Philemon will do will come of his own accord, through the Spirit's leading, because Philemon too has been transformed by the gospel and does not have his past sins held against him in Christ. And in this upside down world of the gospel, a beautiful story of the redemptive and restorative love of Christ is told for us. And because the gospel restores, it really turns what sin made upside down, rightside up!