It has been suggested that Paul’s sending Onesimus back to Philemon, said to be his master, was in disobedience to the Scriptures (cf. Deuteronomy 23:15-16), and a step backward to the Code of Hammurabi (see sections 16 to 20). Yet, nothing is said in Paul’s letter to Philemon of Onesimus being a runaway slave. The interpretation that Onesimus is a runaway slave comes down to us from John Chrysostom (347 AD to 407 AD). So, we must decide whether or not the charge that Paul was siding with Hammurabi over Deuteronomy has any merit.
Paul had greeted people in his epistles by their names, and many of them were slaves or former slaves such as Adronicus and Urbanus (common slave names – see Romans 16:7, 9). He referred to Adronicus as his kinsman and fellow prisoner, and concerning Urbanus, Paul counted him as a fellow worker in Christ. This attitude is in rigid contrast to Aristotle’s philosophy that the “slave has no deliberative faculty at all… (any virtue the slave has) is relative to a master, (and) the slave exists by nature.” Paul’s approach to slavery in the New Testament was absolutely opposite to the status quo and the then current philosophies in the 1st century AD such as Aristotle’s.
As I said above, Paul’s epistle to Philemon never refers to Onesimus as a runaway slave. That interpretation comes to us out of the late fourth or early fifth century AD. Neither is there any language of concern for Onesimus’ safety, since runaway slaves were often brutally treated in the Roman Empire. It can be argued that rather than a master / slave relationship, Philemon and Onesimus were estranged Christian brothers (perhaps biological brothers). Paul seems to be entreating Philemon for Onesimus’ sake in the same manner as the father entreated his eldest son for his youngest son’s sake in the Parable of the Prodigal Son (Luke 15:11-32). Paul considered Philemon as his dear friend and fellow worker (Philemon 1:1, 17), and expects him to go even beyond what Paul asks on behalf of Onesimus. This seems also to be the attitude of the father in Luke 15.
Paul does, indeed, use the word servant in Philemon 1:16, but he also uses the term brother in both the spiritual and physical sense in the same verse. It is entirely possible that Paul used the term servant in the sense of one being without honor, alienated from Roman social circles (and perhaps Philemon’s social circle). No doubt, the elder brother in the Parable of the Prodigal would have no problem receiving his brother back as a hired servant, for a hired servant had no place among the elder brother’s friends (cf. Luke 15:29-30). Yet, Paul asked of Philemon to receive Onesimus as a brother in the Lord and in the flesh (Philemon 1:16).
It seems that Onesimus was not a Christian until he came to Paul (probably at Caesarea, though some believe Rome). Paul specifically states in his epistle that he had begotten Onesimus in his bonds. That is, Onesimus received Christ as his Savior under Paul’s preaching / teaching, while he (Paul) was a prisoner (Philemon 1:10). Consider, as well, the next verse where Paul claims that Onesimus was unprofitable to Philemon prior to Onesimus becoming a believer (Philemon 1:11). How would that be so? If Onesimus was, indeed, a slave, certainly he had to be of some profit to his master. Yet, Paul claims he was unprofitable (G890 – useless). Under what circumstances would someone become profitable to another only because the one became a Christian? If Philemon was Paul’s friend and fellow worker (Philemon 1:1, 17), Onesimus, as an unbeliever, would be unprofitable or useless to Philemon in the work of Christ, even if Onesimus was Philemon’s natural brother. Only by becoming a believer would that change.
On the other hand, even if Onesimus was a slave in Philemon’s household, nothing in the text shows he was a runaway slave. Paul did not act in disobedience to Deuteronomy 23:15-16. Rather his strategy (under this interpretation) would be seen as not forbidding slavery but imposing fellowship. Fellowship works on the heart, and a change in heart would result in a change in relationship. In the New Testament Paul shows himself as a person having a revolutionary approach to injustice and relationships in the first century AD Roman Empire. His teaching transcends racial, social and sexual barriers.
 As I said HERE, this current theme about “making sense of the Old Testament God” is based upon the book: Is God a Moral Monster by Paul Copan. These are my thoughts about his book. He may or may not agree with the impression his book has made upon me, but I thoroughly enjoyed reading what Paul wrote and recommend his book to anyone who is looking for a good read concerning defending our faith.