Anti-Slavery Roots in Christianity- Robert Spencer

"Likewise unacknowledged has been the role that Christian principles played in the abolition of slavery in the West, which was an enterprise unprecedented in the annals of human history. The roots of abolitionism can be traced to the Church’s practice of baptizing slaves and treating them as human beings equal in dignity to all others. St. Isidore of Seville (560–636) declared that “God has made no difference between the soul of the slave and that of the freedman.” His statement was rooted in what St. Paul told the slaveowner Philemon about his runaway slave Onesimus: “Perhaps this was why he was parted from you for a while, that you might have him back forever, no longer as a slave but as more than a slave, as a beloved brother” (Phil. 15–16).
Once it was recognized that the slave had a soul just as did the master, it could not forever be justified that he be another person’s chattel. In the year 649, Clovis II, king of the Franks, married a slave—who later began a campaign to halt the traffic in slaves. The Catholic Church now honors her as St. Bathilda. Charlemagne and others later also opposed the practice in Christian Europe. According to historian Rodney Stark, “slavery ended in medieval Europe only because the church extended its sacraments to all slaves and then managed to impose a ban on the enslavement of Christians (and of Jews). Within the context of medieval Europe, that prohibition was effectively a rule of universal abolition.” And in the New World, when the Spanish conquistadors were energetically enslaving South American Indians, and importing black Africans as slaves as well, their chief opponent was a Catholic missionary and bishop, Bartolome de las Casas (1474–1566), who was instrumental in compelling the Spanish crown to enact a law in 1542 prohibiting the enslavement of the Indians."
-Robert Spencer, First Things (full article)