Americans who strongly embrace the idea that the US is a "Christian nation" are much more likely to believe that "Police treat blacks the same as whites" and that "Police officers shoot blacks more often b/c they are more violent than whites."
Why is that? A couple reasons. A desire to return to a mythic Christian nation means a return to a time when hierarchical relationships between races were clear, with whites at the top.
White Americans who embrace Christian nationalism view out-groups (who are always non-white) as deficient and in need of strong policing to maintain order.