Paul could have commanded Philemon to accept Onesimus back into his household, verse 8; he explains why he did not do so in verse 9 – “…for love’s sake.” He had already commended Philemon’s love for the Lord and for his fellow-Christians (vs. 5), and we could suppose Paul is here appealing to that reputation. More likely, however, is that Paul is concentrating Philemon’s attention on the principle of love, in effect saying that “Christian love should motivate you in the matter about which I am now going to plead with you.” Had Paul simply issued a demand that Philemon accept Onesimus back into his household, that brother might have complied simply because Paul commanded it and not out of any willingness on his own part to forgive Onesimus for his crimes. Even though the outward result might have appeared the same, there would have been no change in Philemon’s heart toward his slave – he would not have done “the right thing” because it would not have been his willing choice to do so. This point matters because God never commands our obedience without ALSO desiring our willing submission. He has the ability to force us to comply with His will – but He always seeks to persuade us, instead.
After “introducing” the subject of his letter (“my son Onesimus,” vs. 10), in verses 11-14 Paul emphasized to Philemon how Onesimus has been changed by becoming a Christian (and, by implication, how his conversion should change their relationship as master and slave). The play on words in verse 11 is based on the meaning of Onesimus’ name (“Profitable” vs. “unprofitable”), and sets the stage for Paul’s request that Philemon allow him to return to Paul (vv. 12-14). We don’t know how Philemon felt toward Onesimus: He may have been angry, especially if Onesimus had stolen something besides himself (such as the funds to travel to Rome). Without discounting the idea that Onesimus had done Philemon wrong, Paul directs his attention to the fact that now their fellowship as brothers in Christ should be the factor that determined how Philemon would respond to Onesimus’ return, not Philemon’s feelings or even the financial harm he might have suffered (cf. vv. 18-19). Paul’s request – to have Onesimus return to serve as his helper, verse 13 – has roots in Philemon’s own earlier demonstrations of love for Paul (vs. 7).
Verses 15-17 offer a valuable insight on Paul’s view of God’s providence, as he suggests to Philemon that his temporary “loss” of Onesimus might very realistically figure into God’s desire for them to become brothers eternally. His mention of fellowship in verse 17 (the word for “partner” is koinonos, which translates as “sharer”) is a way of emphasizing that Philemon should strive to see his “new” relationship with Onesimus in the same way he viewed his relationship with Paul. Paul goes on to reinforce the power of his conviction here by personally offering to repay whatever Philemon might specify to “put right” any harm caused by Onesimus’ actions.