Diarmaid MacCulloch points out in his celebrated book of Christian history the role of Christianity in slave trade (p 868). Christianity stood not only for slavery but also against it. Both groups used the Bible to support their stand.
The Christian church's main justification of the concept of slavery is based on Genesis 9:25-27. Accordingly, when the worldwide flood had concluded, there were only 8 humans alive on earth: Noah, his wife, their three sons and daughters-in-law. In an incident in which Noah's son Ham saw "the nakedness of his father," Noah laid a curse on Ham, which was transferred to Noah's grandson Canaan. "Cursed be Canaan! The lowest of slaves will he be to his brothers. He also said, 'Blessed be the Lord, the God of Shem! May Canaan be the slave of Shem. May God extend the territory of Japheth; may Japheth live in the tents of Shem and may Canaan be his slave'." (Genesis 9:25-27). Christians traditionally believed that Canaan had settled in Africa. The dark skin of Africans became associated with this "curse of Ham." This misunderstanding of the Bible provided the cornerstone for the transatlantic slave trade by Christians.
However the truth is that the religious tradition found in both Old and New Testaments is strongly against slavery. Hebrews were forbidden to kill slaves, force a slave to work on the Sabbath, return an escaped slave, or to slander a slave. It was common for a person to voluntarily sell oneself into slavery for a fixed period of time either to pay off debts or to get food and shelter. It was seen as legitimate to enslave captives obtained through warfare, but not through kidnapping for the purpose of enslaving them. Children could also be sold into debt bondage, which was sometimes ordered by a court of law. The Bible does set minimum rules for the conditions under which slaves were to be kept. Slaves were to be treated as part of an extended family; they were allowed to celebrate the Sukkot festival, and expected to honor Shabbat. Israelite slaves could not to be compelled to work with rigor, and debtors who sold themselves as slaves to their creditors had to be treated the same as a hired servant. If a master harmed a slave, the slave was to be compensated by manumission; if the slave died within 24 to 48 hours, it was to be avenged. (Exodus 20 -23)
Israelite slaves were automatically manumitted after six years of work, and/or at the next Jubilee, although the latter would not apply if the slave was owned by an Israelite and wasn't in debt bondage. Slaves released automatically in their 7th year of service, which did not include female slaves, or did, were to be given livestock, grain, and wine, as a parting gift (possibly hung round their necks). This 7th-year manumission could be voluntarily renounced, which would be signified, as in other Ancient Near Eastern nations, by the slave gaining a ritual ear piercing; after such renunciation, the individual was enslaved forever (and not released at the Jubilee). Non-Israelite slaves could be enslaved indefinitely and were to be treated as inheritable property. (Leviticus 25, Deuteronomy 15).
Jesus identified several issues that hinder a healthy existence of his community. The primary immediate problem Jesus and his movement targeted was the political oppression by the Roman Empire. However Jesus was not seeking a superficial solution but a long-lasting one at a deeper level. He viewed the world as a family with God as the parent and all people as God’s children, which makes them brothers and sisters to one another. Such a relationship rules out all kinds of oppression, including the political oppression by the Romans as well as slavery. If only people have this view of the world in their subconscious mind, it will be exhibited in their actions. Paul also continued such a view. That is why he proclaimed that there is neither Jew nor Gentile, slave nor free, male nor female in Christ. Paul’s advice to the slaves to be submissive to their masters does not mean that he was in support of slavery. He was simply giving a practical advice to the Christian slaves. A change in the social system was what was needed. To achieve that goal, the Christian way was nonviolent. Paul couldn’t advise them to individually revolt against their masters. Following the advice and practice of Paul, early Christianity seemed to have been kind to slaves.
Jesus’ well known advice, “If someone strikes you on the right cheek, turn to him the other also (Mat. 5:38),” seems to be about slavery. At the time of Jesus, striking someone deemed to be of a lower class with the back of the hand was used to assert authority and dominance. If the persecuted person "turned the other cheek," the discipliner was faced with a dilemma. The left hand was used for unclean purposes, so a back-hand strike on the opposite cheek would not be performed. The other alternative would be a slap with the open hand as a challenge or to punch the person, but this was seen as a statement of equality. Thus, by turning the other cheek the persecuted was in effect demanding equality.
However, later the western Roman Christian Empire was for slavery. St Augustine thought that slavery was inevitable. He didn't think that it was the result of the natural laws of the universe - indeed he thought that in a pure world slavery would be quite unnatural, but in our world it was the consequence of sin and the Fall of Man. He argued that slavery was part of the mechanism to preserve the natural order of things. “By nature, then, in the condition in which God first created man, no man is the slave either of another man or of sin. But it is also true that servitude itself is ordained as a punishment by that law which enjoins the preservation of the order of nature, and forbids its disruption. For if nothing had been done in violation of that law, there would have been no need for the discipline of servitude as a punishment.” Although Augustine agrees that slavery is against the original natural order set by God, he argues that slavery was established by God as a result of human sin.
However, his contemporary in the east, Gregory of Nyssa, had a radically different view. Kimberly Flint-Hamilton has made an excellent study of Gregory of Nyssa’s view. In his fourth homily on Ecclesiastes, Gregory denounces slavery on the grounds that the nature of humankind is free. The pleroma, as the fulfillment of God’s will, must be free; it cannot be subservient to any human subdivision. Ownership of one human being over another is therefore antithetical to human nature. God endowed human beings with dominion over all other creatures, but not over other humans, so slavery calls God’s will into question. “Irrational beasts are the only slaves of humankind,” Gregory writes. “But by dividing the human species into two with ‘slavery’ and ‘ownership,’ you have caused it to be enslaved to itself, and to be owner of itself.”
Gregory writes: You condemn a person to slavery whose nature is free and independent, and you make laws opposed to God and contrary to His natural law. For you have subjected one who was made precisely to be lord of the earth, and whom the Creator intended to be a ruler, to the yoke of slavery, in resistance to and rejection of His divine precept. …How is it that you disregard the animals which have been subjected to you as slaves under your hand, and that you should act against a free nature, bringing down one who is of the same nature of yourself, to the level of four-footed beasts or inferior creatures…?
Unlike Augustine, Gregory does not think that slavery was instituted by God. Enslaving fellow beings is a sin according to Gregory. For Augustine, on the other hand, one was merely following the will of God by enslaving others. Unfortunately, Augustine’s line of thought has been followed in the west until the present day. This view provided the corner stone for the Atlantic slave trade when Africans were brought to the US as slaves and human beings were bought and sold like cattle.