Slavery was a much more commonplace and culturally accepted part of society back in the days of Paul. Something that amazes me is the instruction Paul writes in a letter to Titus concerning the witnessing power of slaves.  I mean who would listen or pay attention to one's slave - if you were a slave owner or of higher standing?  Yet Paul says the following to Titus.....
  • Teach slaves to be subject to their masters in everything, to try to please them, not to talk back to them, and not to steal from them, but to show that they can be fully trusted, so that in every way they will make the teaching about God our Savior attractive. - Titus 2:9-10
Paul was saying there is a way for even the slave to make good on his/her new found faith - show it.  Paul mentions for Titus to teach the slaves to "try to please them" [their masters], "not talk back", and "not steal from them". Doing all of this will show they "can be fully trusted" and "in every way they make the teaching [what they have learned and believed about Christ] about God our Savior attractive."

So, here is the message of being a great witness again.  Being salt and light to the world.  However, I hear the Holy Spirit saying through Paul loud and clear it does not matter what station in life you hold - slaves included - if Christ is your Savior make His salvation and the Christian lifestyle - "attractive" - The Redeemed Of The Lord Should Want To Say So