Paul was blunt about it: he could have just told Philemon to accept the escaped slave, Onesimus, back. Why?
Philemon 1:8-9 I could demand it in the name of Christ because it is the right thing for you to do. But because of our love, I prefer simply to ask you. Consider this as a request from me—Paul, an old man and now also a prisoner for the sake of Christ Jesus.
Not that the request would have been easy to for Philemon to refuse. Not once Paul pointed out that Onesimus the escaped slave was now a believer, one of Paul’s spiritual children, and also Philemon’s brother.
Further reading: Onesimus and Philemon