Slavery was well established by the time Paul wrote this letter to the church at Ephesus. It’s estimated that there were 1,500,000 slaves in Italy alone with some wealthy Romans owning upwards of 20,000 each. The owner or master had absolute authority over his slaves, even to ending the slave’s life. Advocating an end to slavery would have caused a revolt. So Paul, by inspiration of God, urged Christian slaves to work within the existing social order, at least for the time being.

http://www.granbychurchofchrist.org/Sermons/pages/2014/Slaves-And-Masters.htm