Comment by wavefunction

Slavery was pretty widespread in Europe until the late Middle Ages, including Christians enslaving other Christians and selling them to slave traders. 10% of the census population of the Domesday Book were slaves, not serfs. This changed when a pope (I can't remember his name off the top of my head) was concerned that Christian slaves owned by Jewish and Muslim slave-owners would convert to their masters' religions. That was just a prohibition against enslaving fellow Christians and selling slaves to Jewish and Muslim slave-traders though.