In the aftermath of the attempted assassination of Donald Trump on July 13, a video with images of Jesus crowned with thorns, blood running down his face, followed by photos of the former president circulated on social media. Days later, at the Republican National Convention, the evangelist Franklin Graham endorsed Mr. Trump from the stage, saying that “God spared his life.”But the idea of Mr. Trump as chosen by God has infuriated those evangelicals who believe that he stands in direct opposition to their faith. Their existence highlights an often-overlooked fact about the American religious landscape: Evangelicals are not a...