For most of the past two millennia, Christian churches have not only accepted slavery, but have also participated in the slave trade and owned human property. The ethics of Christian slaveholding, however, have changed significantly. While Christians owned other Christians without controversy during the late ancient period, Christian churches began to forbid that practice over time.
By the early modern period, it was considered taboo for Christians to own other Christians, although the practice sometimes continued illegally. While some individual Christians, including ministers and members of the clergy, questioned the legitimacy of slavery during the early modern period, it was not until the 18th century that a small minority of Christian churches began to assert an abolitionist stance. Even then, it was deeply contested.
Alternative link: History of the Church and Slavery: Participation, Conversion, Abolition