Question

How did many Europeans justify their enslavement of Africans?

Answer 1

by believing in religious ideas that supported the enslavement of non-Christian people

Answer 2

Europeans were in the past highly patriotic and therefore believed themselves to be the 'superior race ' and thought that this justified slavery .
Also I remember discussion of religion in history class where the teacher said that in the past people were highly religious and interpreted some text from the bible to 'prove ' they were the superior race

Hope this helps