Slavery was an accepted institution in the Roman Empire. Some 35% to 40% of the people of Rome and ancient Italy were slaves. The slave population of the entire Roman Empire has been estimated at just under five million, about 10–15% of the total population. About half of the slaves in the empire were owned by an elite composed of the richest 1.5% of the population.
Roman slavery was not race-based; slaves were drawn from all over Europe and the Mediterranean, including Britain, France, Germany, Spain, North Africa, Greece, Syria, Macedonia, Serbia, the Balkans, Russia, etc. There was some debt slavery, some criminals sentenced to slavery, and some people enslaved through piracy and slave-hunting raids, but most slaves were taken in wars of conquest.
Occasionally, people sold themselves into slavery for personal advancement, since some high government positions were reserved for slaves of the emperor, and senior, educated slaves often administered wealthy households. Frequently, slaves served as the teachers of their masters' children—well-educated Greeks were highly sought after for this purpose---and they became trusted members of the household.
Slaves could be treated brutally, and those employed in agriculture, or worse yet, mining, lived in poor to appalling conditions. But household slaves were often cared for as well or better than free people from the poorer classes. Slavery often provided support during the unproductive years of life--infancy and old age--but sometimes feckless masters would manumit sickly, old or infirm slaves to avoid the expense of caring for them. Gladiator-slaves could be killed in the arena, but sometimes they became celebrities who won their freedom and great wealth. The inequality of slavery might raise barriers between slave and master, but sometimes the institution resulted in strong personal bonds, culminating when the master adopted or married the slave. In short, Roman slavery was ethically (and ethnically) polymorphous.
Slavery was entrenched in Roman society; attacks on the institution of slavery would have been seen as attacks on the Roman state. There had been occasional slave revolts, most notably Spartacus' revolt of 73 BC; these were always brutally suppressed. Had Christianity become associated with an attempt to overthrow the institution of slavery, in likely would have been utterly eradicated, rather than intermittently persecuted.
The Bible writers do not directly attack slavery. They seek to make society better by softening hearts, not by rearranging social institutions. Paul admonished slaves to obey their masters as they would obey Christ (Eph. 6:5-8; Col 3:22-25; Titus 2:9-10). He also admonished masters to treat their slaves with justice and fairness, because the Master of all watches from heaven (Eph. 6:9; Col. 4:1).
Paul also emphasized spiritual equality; the master was not more important to God than the slave. Christ abolished all spiritual barriers based upon race, class, sex, or station in life: “Here there is no Greek or Jew, circumcised or uncircumcised, barbarian, Scythian, slave or free, but Christ is all, and is in all (Col 3:11; see also Gal. 3:28).
Unity in Christ gave rise to behavioral obligations:
“Therefore, as God’s chosen people, holy and dearly loved, clothe yourselves with compassion, kindness, humility, gentleness and patience. Bear with each other and forgive whatever grievances you may have against one another. Forgive as the Lord forgave you. . . . Slaves, obey your earthly masters in everything; and do it, not only when their eye is on you and to win their favor, but with sincerity of heart and reverence for the Lord. . . . Masters, provide your slaves with what is right and fair, because you know that you also have a Master in heaven” Col. 3:12-4:1 (NIV).
Christianity’s impact on slavery is best illustrated in Paul’s letter to Philemon, a slave-owner of Colosse (in ancient Phrygia, now western Turkey) whom Paul had won to the faith. Onesimus was Philemon's slave, but had stolen money and run away, eventually making his way to Rome, where Paul, under house arrest, awaited a hearing before the emperor. Onesimus was converted to the new Christian faith, and became a friend and helper to Paul. Eventually, Paul sent him back to Philemon, carrying the letter that is now in the New Testament.
The letter has a clear message to Philemon about how he is to treat Onesimus:
“Therefore, although in Christ I could be bold and order you to do what you ought to do, yet I appeal to you on the basis of love. I then, as Paul—an old man and now also a prisoner of Christ Jesus— I appeal to you for my son Onesimus, who became my son while I was in chains.”
Philemon is to treat Onesimus not as a runaway slave, but as the son of the dear friend who brought Philemon to Christ.
Paul engages in some wordplay on the meaning of Onesimus’ name (“useful”), to emphasize that “Mr. Useful” used to be useless, but not anymore:
“Formerly he was useless to you, but now he has become useful both to you and to me.”
A converted Onesimus now lives up to his name.
“I am sending him—who is my very heart—back to you. I would have liked to keep him with me so that he could take your place in helping me while I am in chains for the gospel.”
Philemon is to think of Onesimus not as having run away, but as having performed a Christian mission on behalf of Philemon, in Philemon's stead.
“But I did not want to do anything without your consent, so that any favor you do will be spontaneous and not forced. Perhaps the reason he was separated from you for a little while was that you might have him back for good—no longer as a slave, but better than a slave, as a dear brother. He is very dear to me but even dearer to you, both as a man and as a brother in the Lord.”
The message is clear: Philemon is not to think of, or treat, Onesimus as a slave. He is to think of him and treat him as a brother, and not just as any brother in Christ, but as he would treat Paul himself, if he were there:
“So if you consider me a partner, welcome him as you would welcome me.”
Philemon is not even to ask Onesimus to pay back the money he stole.
“If he has done you any wrong or owes you anything, charge it to me. I, Paul, am writing this with my own hand. I will pay it back—not to mention that you owe me your very self.”
What are a few shekels between brothers, especially when Philemon now has the gospel and eternal life, and Paul told him about that salvation?
“I do wish, brother, that I may have some benefit from you in the Lord; refresh my heart in Christ. Confident of your obedience, I write to you, knowing that you will do even more than I ask.”
These are not just suggestions; this is how Philemon must treat Onesimus. This is how it must be between brothers in the Lord Jesus.
So we see that, although Paul did not challenge slavery's legality, he obliterated its controlling assumptions. As practiced according to the guidelines of the Epistle to Philemon, slavery is unrecognizable. The common faith of Paul, Philemon and Onesimus totally overshadows and negates the law of slavery. Everything significant about the way these men will treat each other is determined by their Christian faith, not by Roman law, custom, or practice.
Now the Lord is the Spirit, and where the Spirit of the Lord is, there is freedom. I Cor. 3:17