Abraham circumcised the slaves within his house. But OT slavery existed only as a mercy: those reduced to such were in that condition because they had not been killed in holy war or like reasons. OT slavery had to do, in other words, with the covenant people under the Old economy and those reasons ceased when that economy ended (in the Coming of Christ).
Greek and Roman slavery was different, of course, and neither the Lord nor his apostles called for its overthrow because this would have reduced the gospel to a politcal revolution. They did tell masters and slaves how to behave under it, while making it clear that one of the effects of the gospel would mean its end (as Paul in Philemon). Much work has been done in the last decade about how Christianity brought this slavery to an end. Here are a couple of footnotes from an article soon to be published that address that:
"As seen in the practices of Christians in the early church, in A. J. Harrill
The Manumission of Slaves in Early Christianity (Tubingen: J. C. B. Mohr/Siebeck, 1995). Though opposition to slavery itself, as opposed merely to slavery’s abuses, was long in coming, as seen in Trevor Dennis, “Man Beyond Price: Gregory of Nyssa and Slavery,” in
Heaven and Earth: Essex Essays in Theology and Ethics, ed. Andrew Linzey and Peter J. Wexler (Worthing, West Sussex: Churchman Publishing Limited, 1986), it was Christianity, or Christendom, at least in part, that brought slavery to an effective end between the fourth and tenth centuries, with serfdom developing in seignorialism and feudalism subsequent to slavery’s diminution. Though Kyle Harper, in
Slavery in the Late Roman World, AD 275-425 (New York: Cambridge University Press, 2011), showed that slavery lasted deep into the Christian era, in his most recent book,
From Shame to Sin: The Christian Transformation of Sexual Morality in Late Antiquity (Cambridge, Mass.: Harvard University Press, 2013), he shows that Christianity’s strict moral code was particularly sympathetic to the sexual exploitation of the slave. So Christianity played an important role in reforming and ultimately ending ancient slavery."
All this is to say that when, in the modern era, Christians played no small part in introducing a racially-based slavery that arose with the development of early capitalism, it is to our eternal shame. Presbyterians did, as others here have noted, recognize this, both in the era of the Revolution and afterwards, most notably in the famous statement condemning slavery in the 1818 GA, but we were quite complicit with an institution for far too long that not only had no biblical warrant but ran directly counter to the biblical commands of how we are to treat fellow humans made in the image of God. Our own Dr. Ritchie has written well in this area.
So--bottom line--Trevor: Had you lived in Paul's day and had slaves (and thus not been a Baptist

), I would say that the Apostle would intend for you to baptize them, though, at the same time, he would prefer that you not hold them as slaves, especially if they are Christians, and that you endeavor to manumit them (Philemon). This same Paul would have rejoiced to see slavery finally eliminated and lamented its racially-based return in the modern era, calling on all Christians to have nothing to do with that. So, as MW said, if you had slaves now, he would surely tell you to manumit them. Bruce B. has also had a lot of good stuff to say about this and unless and until the whole Reformed and Presbyterian Church head in this direction (there are still those among us who seek to justify modern slavery), we remain in trouble, a kind of corporate failure to be convicted of sin.
Modern slavery was sin--man-stealing (I Timothy 1:10)--and some Presbyterians saw it all along (like the Covenanters), while others of us have had to play catch up and some, amazingly, have still not caught up and continue to justify it. So, as you can see, much conviction for sin still needed even among the saved.
Peace,
Alan