Christianity. There were plenty of racists back in the day who used the bible to support their views: https://www.google.com/search?q=biblical+support+for+racism

Though you could step beyond my/the parent comment's literal words to get to the gist of the point: While they deserve some sensitivity because they are a product of their religion/upbringing/culture, they don't get a free pass (google "moral relativism" if you want to see a mess of discussions about this).