I realize there's a HUGE scope to this question, so I'll try and add some context.

It seem like there's an identifiable progression in the west (with some notable deviations and exceptions) from the ancient Greek/Roman view on slavery to Medieval Serfdom and then finally to abolitionism/the Colombian exchange. What changes in the west account for this? Can it all be explained by the rise of Christianity/the eventual birth of the western concept of race, or is there something else at play?