Dick Lucas puts it very well when he says that
Richard D. Phillipsfor Paul there is a clear line of demarcation between the world and the church. To him the world is essentially unchanged and unchanging. In every generation the seeds of decay and disruption are simply reproduced. Human bondage in one form or another will continue in a fallen world, bringing with it ever new examples of suffering, loss, indignity and shame. To live in this world, Christians cannot avoid living within social structures that are unjust and unequal. Yes, their presence within it should be an assault for life; they should help to change the structures. And yet, whether one is born an Onesimus or is born a Philemon, the only secret of freedom is found through faith in Jesus Christ.That is Paul's point.1
15 For this perhaps is why he was parted from you for a while, that you might have him back forever,