I discovered that Christian opposition to the slave trade goes back yet further. In "The Stories of English", by David Crystal, he quotes a sermon in Old English, by Bishop Wulfstan, composed in 1014:
It is also no wonder that things are going badly for us since we now know very well that many men of long ago did not care very often what they did in word or deed. And the people, as it can seem, became very corrupted through numerous sins and through many evil deeds: through deadly sins and through crimes, through greed and through gluttony; through theft and through robbery; through slave-trafficking and through pagan vices, through deceits and frauds ...In actual fact, he wrote it in Old English - "mannsylena" is the word translated by Crystal as "slave-trafficking". However, in the 11th Century, at a point when the available documentary evidence is quite slight, it is worth noticing that even then slave-trafficking was clearly understood within the church to be worthy of reproach.