Some Catholic writers claim that it was not until 1890 that the Roman Catholic Church repudiated slavery. A British priest has charged that this did not occur until 1965. Nonsense!
As early as the seventh century, Saint Bathilde (wife of King Clovis II) became famous for her campaign to stop slave-trading and free all slaves; in 851 Saint Anskar began his efforts to halt the Viking slave trade. That the Church willingly baptized slaves was claimed as proof that they had souls, and soon both kings and bishops—including William the Conqueror (1027-1087) and Saints Wulfstan (1009-1095) and Anselm (1033-1109)—forbade the enslavement of Christians.
Since, except for small settlements of Jews, and the Vikings in the north, everyone was at least nominally a Christian, that effectively abolished slavery in medieval Europe, except at the southern and eastern interfaces with Islam where both sides enslaved one another's prisoners. But even this was sometimes condemned: in the tenth century, bishops in Venice did public penance for past involvement in the Moorish slave trade and sought to prevent all Venetians from involvement in slavery. Then, in the thirteenth century, Saint Thomas Aquinas deduced that slavery was a sin, and a series of popes upheld his position, beginning in 1435 and culminating in three major pronouncements against slavery by Pope Paul III in 1537.
It is significant that in Aquinas's day, slavery was a thing of the past or of distant lands. Consequently, he gave very little attention to the subject per se, paying more attention to serfdom, which he held to be repugnant.
However, in his overall analysis of morality in human relationships, Aquinas placed slavery in opposition to natural law, deducing that all "rational ...