Paul said that slaves should be obedient to their earthly master as if he was Jesus Christ. These slaves should do the will of God as servants from the heart with an enthusiastic service. They are rendering a service to God, not to men. Whatever good they did, they will receive the same again from the Lord. This also goes for the free slave owner. Masters were not to threaten their slaves. Both the free master and the slave serve the same master in heaven where there is no partiality.