Slavery was common in the Roman world. Prisoners of war often became slaves, many were born into slavery, and some chose slavery temporarily to pay off debts. A large part of the population, including many Christians, were slaves or former slaves (see Acts 6:9). So, why did the apostle Paul not speak against slavery?
There are two reasons. First, slavery in the Roman world was different from the slavery familiar to the modern western world. It was not based on race and was rarely lifelong. Most slaves expected freedom by age thirty. Some people even sold themselves into slavery to increase social status. Some slaves were well-educated and held important roles in their households and society. Many had friendly relationships with their masters. In the Roman world, Christians like Philemon did not feel guilty about owning slaves.
Second, Paul did not oppose slavery because it was not his mission to change society's structures but to build the church. He wanted people to convert and transform spiritually. For Paul, whether a Christian is a slave or not is not very important, just like other external circumstances. The key is serving Jesus Christ, no matter one's life situation. In other words, Paul focused more on Christian life and witness than on physical freedom.
Even slaves can do their work as an act of worship (Ephesians 6:5–8; Colossians 3:22–24; compare 1 Timothy 6:1–2). When Paul speaks to Christian slaves, he encourages them to see their situation as a calling where they can serve Christ. Christ is their true master, and in him, they are truly free (1 Corinthians 7:20–24).
Every Christian, whether slave or free, has been freed from the worst kind of slavery, slavery to sin. They can now experience true freedom by living as a servant of God and righteousness (Romans 6:6–22). For Paul, this is the only kind of slavery and freedom that truly matters.
Passages for Further Study
Romans 6:6–22; 1 Corinthians 7:20–24; 9:19; Ephesians 6:5–8; Colossians 3:22–24; 1 Timothy 6:1–2; Philemon 1:8–21