Another critical question is whether we could have ever come upon the idea that slavery is wrong if Paul's point had not been made. People became slaves historically because of military defeat. Slaves were the spoils of battle. On a polytheistic view, if you lose a war, your god was defeated by the other country's god, and so they had the right to treat you as human refuse. Paul says that the slave has to be treated in certain ways because you and the slave are both creatures of the same God, and God plays no favorites.
But what if we had gone from a polytheistic view to the view that we weren't created at all, but were spat up by evolution. Wouldn't it be natural to think that a country who had just won a war was the country that was "selected for," as if were, and could do what it wanted to do with the people of the country that lost? It's a dog eat dog world, and natural selection supports those who can enslave others and get them to do hard labor. Did the free people of Egypt build the Pyramids? How did ships cross the Mediterranean sea. Could you have gotten a galley full of rowers with volunteer labor? Could a convincing case against exploitation have been made without the kind of theological appeal that Paul makes?
Of course, nowadays we all hate slavery (though believe me, it's still around!) But it came naturally for a lot of people to treat it as perfectly acceptable. We all imagine that if we had been in, say, the antebellum South, we would have seen through the detestable practice. But the moral force of the abolitionist arguments came from their Christianity. It did not come from secular humanism.