How to respond:
Start by addressing the verses that will most likely be brought up (Ephesians 6:5, Colossians 3:22, 1 Peter 2:18). Admit that, on face value, it does seem to indicate that the Bible does seem to support certain slave-owner relationships. Show that slavery in the Bible is not the same as our modern conception of the practice. Our modern view is most likely based in the ownership model common in the first century of America’s history.
The Biblical model, in both Old and New Testaments, is very different. Jewish and Roman slaves were bond servants who were given significant legal status. Very few were slaves for life and there was no little to no forced slavery based on race. Most slaves were prisoners of war who would have else wise been executed as enemies of the state.
Even with this distinction in place, Paul writes to Philemon that his runaway slave should be treated as an equal (Philemon 1:17) therefore instructing the elimination of the slave-owner relationship.