The entire Atlantic economy in the 17th and 18th centuries was based on the enslavement of (mostly) non-Christian Africans. As this lecture will show, slavery was at first a practice which many missionaries hoped to mitigate; then a vast reality with which they felt they had to work, and in which they were deeply implicated; until, finally, it became an intolerable obstacle. Spiritual and worldly matters could not, despite the missionaries’ best efforts, be kept apart: a lesson with enduring consequences.