Is Paul, then, condoning slavery as an acceptable societal institution?
Of course not! But raising the question does give the opportunity to look at a basic principle of biblical interpretation. Briefly stated, this principle is that the cultural context which the gospel encounters is descriptive not prescriptive.
The gospel makes a home for itself in all cultures but is captive to none. What we have in the New Testament are descriptions of how it made a home for itself in the various cultures it encountered in the Greco-Roman world of the first century. Our task is not to replicate the way the gospel took shape then and there but to carefully observe the intention and direction the gospel reshaped those relationships and discern how it can reshape ours in the same direction and with the same intention in the changed and changing situations we find ourselves in.
Far, then, from accepting or condoning slavery, Paul objects vigorously to the way it forms relationships between human beings. He sees clearly that the oppressive, dehumanizing, destructive is contrary to everything he had learned and experienced in the gospel. In Colossians, we find Paul's theological exposition of the reign of the victorious Christ over all things on which he builds here. It is hard to deny that the apostle has at least in principle recognized the unacceptability of an institution that denied Christ's victory and rejected God's intent for human life. What he could do about that, on a macro-level was very little (as we have seen). But what he could do in re-shaping one master-slave relationship in light of the gospel, he did. And the transformation he envisioned in that relationship he also envisioned for the church since he made them co-recipients of his letter to Philemon.
I believe that Paul's pastoral work in the midst of the Colossian congregation, included in our Bibles, set a trajectory in motion that with time and opportunity stirred the church to oppose slavery and all such similar abominations in the name of Christ and the gospel. Abolition, then, was the proper and necessary action of the church following the lead of the apostle Paul here in Philemon. If we misread Paul so as to have him tacitly or overtly supporting slavery, we cut off our nose to spite our face!
The church, of course, has not always followed that lead with vigor and intention. In fact, it has too often stood in the way of living out the liberating fullness of the gospel. This sprang, in part, at least, from a failure to read the Bible descriptively rather than prescriptively. To repeat, we read the Bible to discover the ways and the directions in which the gospel takes what is (our cultural setting) and moves it toward what should be.
Incarnation, giving new flesh to the life of Christ in us, is the way transformation happens. Living within the givens of the host culture, Christians would "seed" their relationships with the values and virtues of the gospel. And it bears fruit by filling the forms indigenous to its host culture with new content. Changes them from inside out, as it were. At some point in this process the impetus of the gospel leads to changes in the forms of relationships too. At no point in history is this process ever perfected and, thus, frozen into a form that must stay the same forever. Thus, the church is always in the process of seeking to live out its gospel as best it can and discovering new ways and shapes faithful living may generate.
Our efforts at incarnation brings us round full circle back to reconciliation. For we can incarnate the gospel in our time and place only because of THE Incarnation of Jesus Christ and the victory his life, death, resurrection, and ascension have won for us. Reconciliation is surely the center of Paul's understanding of what God is up to in his world.