In a society where slavery was normal, an escaped slave who had stolen money (probably to help his escape) had even fewer rights than no rights at all.
So Paul’s gentle pleading with Philemon to welcome this escaped slave as he would welcome Paul himself was revolutionary. And why did he do it? Because Onesimus the escaped slave was now a believer, one of Paul’s spiritual children.
Philemon 1:17-18 So if you consider me your partner, welcome him as you would welcome me. If he has wronged you in any way or owes you anything, charge it to me.
And just to drive home the message, Paul reminded Philemon of how much he owed Paul:
Philemon 1:19 I, Paul, write this with my own hand: I will repay it. And I won’t mention that you owe me your very soul!
There was really nothing Philemon could do, other than agree.
Further reading: Onesimus and Philemon