Onesimus as Philemon's Useful Substitute (1:12-13)
In justifying his appeal that Philemon take Onesimus back into his home, Paul is well aware that Onesimus's solicitation of his support has created a delicate situation: a non-Christian slave has come to the apostle, in prison, to gain some advantage over his Christian master. He must make reconciliation possible by asking a trusted colleague for a radical act: the manumission of his slave (see introduction). The motives to which Paul appeals are deeply rooted in Philemon's spiritual obligations to him as spiritual father.
First, Paul is not sending Onesimus back to Philemon, who is probably not a Roman citizen, for legal reasons to secure his official consent (see above; here I differ from O'Brien, Wright and others), but rather because he is Paul's very heart (or splanchna, which means compassion; see commentary on verses 6-7). For this reason, Paul states his personal preference that Onesimus remain with him in prison.
While it is not clear why Paul should speak of Onesimus as his very heart, two possibilities seem best: (1) the imprisoned Paul may be designating Onesimus as his agent, so that by delivering this letter to Philemon, Onesimus can be the mediator of Paul's heart (or compassion) in his absence (O'Brien 1982:293); or (2) Onesimus himself may be a source of compassion for the imprisoned Paul. I am inclined toward the second meaning, since it best explains Paul's hesitancy in sending Onesimus home. He has been Paul's faithful servant (v. 13) and has been his only source of compassion while he is in chains for the gospel. Yet when Paul prayed for Philemon's compassion in verse 7, heart was coupled with koinonia and indicated the requisite capacity of a nurturing congregation. Perhaps Paul implies the same here: Onesimus is useful to Philemon as one who has the capacity for compassion and therefore can help Philemon and his household church reach its ideal of koinonia (see Phil 1:5).
A second and perhaps more critical reason is located in the purpose clause at the end of verse 13, where Paul says that Onesimus's ministrations are so that he could take [Philemon's] place. Paul's language is of substitution, suggesting he "assumes that Philemon would have wished to attend to Paul's needs personally if such had been possible" (Harris 1991:264). Such are the obligations of the spiritual son to his father and of the indebted client to his patron. Moreover, Paul has already expressed his confidence in Philemon's capacity and readiness to dispense compassion in order to build Christian partnership (v. 7). Rather than stressing Onesimus's usefulness as his substitute or proxy, Paul underscores Onesimus's usefulness as Philemon's substitute in the important work of heartfelt servanthood (see commentary on vv. 18-19). Ironically, Onesimus continues to function as Philemon's slave, but now as Paul's servant.
Finally, Paul's word choice for helping (diakoneo) is striking because it comes from a different word for "slave" from the one he then uses in verse 16 (doulos). According to O'Brien, Paul uses words from the diakoneo family when speaking of gospel ministry (as in Col 1:7, 23, 25; 4:7, 17) rather than of the degrading work associated with a prisoner's slave (1982:294). But it seems to me that Paul's intent is more ironical: Onesimus is no longer a slave (doulos, v. 16), even though his labor of love could well be seen as degrading work; rather, he is Paul's minister and therefore a useful substitute for Philemon. On this basis, then, Philemon's manumission of Onesimus can be rightfully granted.
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