"Many modern readers assume that slavery in the New Testament is equivalent to the race-based slavery of the African slave trade. While not defending the Greco-Roman institution of slavery, Tim Keller and Don Carson explain why it’s important not to equate it with the race-based slavery that we may be more familiar with."He is pulling together comments from Tim Keller and Don Carson (via Lee Strobel).
Timothy Keller, Every Good Endeavor: Connecting Your Work to God’s Work (New York: Dutton, 2012), 213–14, 280–83. (This new book looks interesting - perhaps more another time!)
Lee Strobel, The Case for Christ: A Journalist’s Personal Investigation of the Evidence for Jesus (Grand Rapids: Zondervan, 1998), 166–69):This is a great reminder of the importance of not making assumptions about what the Bible writers meant when they use words we think we understand - especially 'hot button' words like 'slavery'.
Definitely worth reading the whole article! (http://andynaselli.com/keller-and-carson-greco-roman-slavery-race-based-slavery).
(You may also be interested in my post on the struggle to eradicate modern slavery.)