The bible not only approves of slavery in many passages, it gives a description of how to treat them.  

When a slave owner strikes a male or female slave with a rod and the slave dies immediately, the owner shall be punished. But if the slave survives a day or two, there is no punishment; for the slave is the owner’s property. (Exodus . 21:20-21)

In the New Testament Jesus never shows disapproval of slavery and in some cases shows acceptance or approval. 

A disciple is not above the teacher, nor a slave above the master (Matt. 10:24)
Who then is the faithful and wise slave, whom his master has put in charge of his household, to give the other slaves their allowance of food at the proper time? Blessed is that slave whom his master will find at work when he arrives. (Matt. 24:45-46)

Early Christian history also showed approval. There was almost universal approval of slavery among church leaders. Christians vigorously defended slavery as being

directed by God.

The slave should be resigned to his lot, in obeying his master he is obeying God… (Saint John Chrysostom)
…slavery is now penal in character and planned by that law which commands the preservation of the natural order and forbids disturbance. (Saint Augustine)

In the 18th century it was clear that Christianity had freed us from the slavery of sin, not from earthly and physical slavery:

The Freedom which Christianity gives, is a Freedom from the Bondage of Sin and Satan, and from the Dominion of Men’s Lusts and Passions and inordinate Desires; but as to their outward Condition, whatever that was before, whether bond or free, their being baptized, and becoming Christians, makes no manner of Change in it.  (Edmund Gibson, Anglican Bishop in London)

American slavery was no different. It, as an institution, received theological support from various religious leaders.

Christianity actually became a way to add value to slaves in America:

…when southern clergy became ardent defenders of slavery, the master class could look upon organized religion as an ally …the gospel, instead of becoming a mean of creating trouble and strive, was really the best instrument to preserve peace and good conduct among the negroes. (Kenneth Stamp in The Peculiar Institution)

As northerners cried for abolition of slavery the southern political and theological leaders found an ally in the Bible and the history of Christianity.

…Jesus Christ recognized this institution as one that was lawful among men, and regulated its relative duties… I affirm then, first (and no man denies) that Jesus Christ has not abolished slavery by a prohibitory command; and second, I affirm, he has introduced no new moral principle which can work its destruction… (Reverend Thomas Stringfellow, a Baptist minister from Culpepper County in Virginia, )

People still used the bible after the slaves were freed to suppress the black people. The ‘mark of Cain’ is frequently referred to as a curse from God, and as what was responsible for black people being black.

Few people are aware that the KKK was organized as a Christian organization. In the earliest days of organizing, the KKK regularly recruited from churches, attracting not only the common parishioner but members of the clergy as well.

It truly astounds me that there are so many black churches in America today, with the sordid history of Christianity clearly evident and not that so long ago.