The quiet part they’re not saying is that in their minds the fact he doesn’t “believe” in their precious book (not that they follow it much themselves) invalidates him both as a politician and a person. It’s just a more “politically acceptable” excuse for their irrational fear/hatred of someone different from them - they’re afraid all the bullshit they’ve been programmed to believe all their lives about outsiders being out to get them is now going to come to fruition, and they’ll pick at the most absurd things to claim it’s true rather than admit they might be wrong.