Today's reflection from Catholic Exchange is worth repeating.
So if you consider me your partner, receive him as you would receive me. If he has wronged you at all, or owes you anything, charge that to my account. - Philemon 17-18
Onesimus flew the coop on his master, Philemon. As one might expect, he wasn't terribly fussy about private property since Philemon had owned Onesimus as a bit of private property. But something happened to Onesimus. He went to Paul perhaps for protection and, through him, was introduced to Jesus Christ. Instead of a bunch of lectures about "knowing his place" Onesimus found he was a man made in the image of Christ, his human dignity affirmed, and most amazing of all the ability to forgiven and return to Philemon, "no longer as a slave but more than a slave, as a beloved brother" Phil 16.
As to his sins of theft, Paul did a typically Christlike thing: he took them upon himself, laid no burden on Onesimus's shoulders and wrote Philemon with the very strong hint that he should set him free for what man holds his own brother as a slave?. In all this, Paul showed a salient fact: Forgiveness is not just vertical between God and man. It has a horizontal dimension. God's forgiveness healed the relationship, not merely between God and Philemon or God and Onesimus, it healed the relationship between Philemon and Onesimus. It still heals the Church today.
Just a Word of Encouragement from Mark Shea and Jeff Cavins