It never occurred to anyone, in any culture, to consider slavery immoral, it was part of the taken-for-granted institutions of society, and the Bible reflects that. Neither the Old nor the New testament encouraged social revolution. 

Slavery was taken for granted in pre-Christian societies, and continued to be taken for granted after Christianity.

However, by the 17th century, the Christian principles of charity, respect for the poor, injunction to "do unto others as you would have them do unto you", and "In God there is no man or woman, slave or freeman", etc., led some of the more strict Christians, the Quakers, to not only question the morality of slavery, but to start the abolitionist movement.

This is the first time in world history that people organised a social movement to stop what they saw as an injustice against another group, not their own. It could not have happened, and did not, without the basis of Christian principles as one of the factors.