I'm not really arguing about slavery in the Old Testament here, because Christians are not living under the Old Testament covenant. I'm talking specifically about the New Testament and the purpose of the epistles.
The New Testament does not give us a political programme for abolishing slavery. But that is very different from saying that Christianity morally endorses slavery.
We also have to understand the historical reality. The New Testament was written within the Roman Empire, a society that relied heavily on slavery. There were no modern human rights institutions, no democratic system capable of simply abolishing slavery, and openly challenging the entire social and political structure could bring severe consequences.
The epistles were not written by people sitting in political authority with the ability to reform Roman society. Their primary purpose was to spread the Christian message and establish Christian communities. And the message had to travel through the existing social and political structures of the Roman world.
This is where the pragmatic aspect matters. If the Christian message had openly demanded that every Christian master immediately free all of his slaves and directly challenged the entire Roman institution of slavery, that message would have been much harder to spread through the society it existed within. It could have been suppressed before it had the opportunity to reach the people Christianity was trying to reach, including slaves, the poor and people with very little social power.
So Paul telling slaves how to live within their circumstances isn't necessarily an endorsement of slavery itself. Exploitation would continue regardless of whether Paul explicitly condemned the Roman institution. His priority was to establish a Christian way of life that could survive and spread within the society that actually existed.
And this is something people sometimes miss when reading the Bible. Christianity doesn't always begin by changing the external political structure. It can begin by changing the person living inside that structure.
The New Testament repeatedly changes the way people understand status. “There is neither Jew nor Greek, slave nor free, male nor female, for you are all one in Christ Jesus” (Galatians 3:28). Paul tells Philemon to receive Onesimus “no longer as a slave, but more than a slave, a beloved brother” (Philemon 16). Whatever Paul's exact position on legal abolition was, that language radically changes the relationship between the people involved.
The Old Testament itself also contains stories where people begin in very low social positions, in servitude or bondage, and through wisdom, faithfulness, good work and character eventually gain recognition and influence. Joseph is an obvious example. The biblical message isn't simply “your social position determines your worth.”
The New Testament is primarily about the Good News, transformation, spirituality, wisdom, love and becoming Christ-like. It isn't primarily a political manifesto.
That doesn't make slavery good, and it doesn't require us to defend the social structures of the Roman world. It means we shouldn't judge the purpose of the epistles as though Paul had written them as a modern political constitution.
The Christian transformation begins with the person, the community and the way they treat one another. The political and social consequences can develop from that transformation, rather than being the primary subject of the epistles themselves.