Many who don't know Christ read the word "slave" in the Bible and immediately focus on the atrocity of the slave trade in the United States, However, it's important to understand context. Timothy Keller, senior pastor of Redeemer Presbyterian Church in New York, wrote a wonderful book called The Reason for God. In it, he expounds on this topic:
In the first-century Roman empire, when the New Testament was written, there was not a great difference between slaves and the average free person. Slaves were not distinguishable from others by race, speech, or clothing ... From a financial standpoint, slaves made the same wages as free laborers, and therefore were not usually poor. Also, slaves could accrue enough capital to buy themselves out. Most important of all, very few laves were slaves for life. Most could reasonably hope to be manumitted within ten or fifteen years ...So in the time Paul wrote about masters and slaves, his words were really more about our modern employer and employee. Employees are paid fair wages and many work at the same job for ten or fifteen years.
Employers should, therefore, look at today's verse as an exhortation: Just as they are "masters" of employees, they serve the Master in heaven. And just as they want to be treated by God with "justice and fairness," they should treat their employees in the same way.
Do you supervise others? Take these words to heart.