An interesting angle on this is to perhaps not view the group of 'evangelicals' as a discrete subset of the US population but to overlap that (Venn Diagram) group with ~white nationalists / far right / conservatives.  Noting the list of states of inter-racial marriage, the states with the lowest % are, aside from a relatively isolated part of PA, all in the South.

My take on this is that many (not just Christians) used their religion to justify their domination and appropriation (theft) of others' lands based on race, ethnicity and faith / religious beliefs.  Just look at the Dominionists of today who promote Manifest Destiny v2017 (Bachman, Palin, Perry.. and vp Pence, along with a multitude of evangelical preachers- Dobson, Hagee, Robertson, Graham jr; revisionist 'historian' David Barton, and many others).   

https://www.thedailybeast.com/dominionism-michele-bachmann-and-rick-perrys-dangerous-religious-bond