[13-17] Paul wanted to keep Onesimus with him, but he knew that it wouldn't be right to do so. So, Paul sent Onesimus back to Philemon with the assurance that Philemon would do the right thing; send Onesimus back to Paul to assist Paul in the ministry of the gospel. Paul wants Philemon to understand that Onesimus is no longer a slave [but a beloved brother]. Paul pleads with Philemon to welcome Onesimus just as he would welcome other believers in Jesus Christ.
[18-19] Paul tells Philemon, "If he has wronged you or owes you anything... charge it to me." In other words, Paul was willing to pay the penalty for the wrong that Onesimus had done. This is such a beautiful picture of what Jesus Christ has done for mankind. Jesus bore the penalty for the wrong that each one of us has done.
[20] Paul asks Philemon to do this favor [for the Lord's sake]. In other words, this will be encouraging to me, to Onesimus and it will please the Lord. Philemon had every right to punish Onesimus [even to the point of death] under Roman law. Paul pleads for Philemon to offer forgiveness and mercy.
