I don't know if Christianity is the main reason, but the basis for all religion is to hold power over the people that believe in it. I grew up a Catholic but I was never taught in my home or in the church that people of other ethnicities, skin colors, etc were bad or to be avoided. I think white supremacists are just afraid there won't be a majority of white people anymore. People fear what they do not understand. I also think hate is handed down from generation to generation within families. I haven't watched the movie you mention but I will put it on my list.

The lack of trust in science is one factor and prejudice is a big one.