St Paul accepted that slaves existed in the Roman Empire. St Paul taught that slaves were to be like Christ.
Slaves, obey your earthly masters with respect and fear, and with sincerity of heart, just as you would obey Christ. Obey them not only to win their favor when their eye is on you, but like slaves of Christ, doing the will of God from your heart. Serve wholeheartedly, as if you were serving the Lord, not men, because you know that the Lord will reward everyone for whatever good he does, whether he is slave or free.
And masters, treat your slaves in the same way. Do not threaten them, since you know that he who is both their Master and yours is in heaven, and there is no favoritism with him. Ephesians 6:5-9 (New International Version)
Later on the Christians opposed slavery as their masters could treat them as they liked, sometimes beating them to death. Slave owners could do what they liked to slaves, and were not answerable to any authorities for this. Christian faith taught that all were answerable to God in Christ for their actions, hence, mistreatment of slaves was wrong. In history, Christianity brought about the end of slavery in the Roman Empire.