Recently, Beneath the Tangles featured a very long and well-written post on the topic of slavery in The Rising of the Shield Hero. It is worth your time to read when you have a good chunk of free time:
Slavery is a very interesting topic in regards to Christianity, because the Bible never condemns it in explicit terms. This has led to epochs where rulers and nobility saw slavery as permissible, especially in the Age of Exploration and when the wars between Christendom and Islam became more advanced. Thus, the papacy had to condemn the practice several times in encyclicals and statements in the years 1462, 1537, 1639, 1741, 1815, and 1839. (See Slavery: A Problem in American Institutional and Intellectual Life by Stanley M. Elkins.) I might also add the 1435 encyclical commanding that Canary Islanders be freed from the condition of slavery. That slavery could be countenanced is rather odd when one considers that Medieval society had made great strides in eliminating slavery with its borders so that it was virtually non-existent by the 11th century, which coincides with the end of the Viking Age.