I think we have to remember , as far as the apostle Paul is concerned, he was not interested in social Justice, he was interested in the conversion of souls. Therefore, he would not have been interested in freeing slaves, per se, as slavery was ubiquitous, as Dr Brown said. Paul's interest was morality and salvation. why would he condemn it? It was neither illegal nor immoral so long as the slave was not mistreated. However, sex outside of marriage in any form was wrong.