Journalists and scholars have spent decades examining the influence of conservative religion on American politics, but we largely missed the impact conservative politics was having on religion itself. As a progressive evangelical and journalist covering religion, I’m as guilty as any of not noticing what was happening. We kept asking how white conservative evangelicals could support Mr. Trump, who luxuriates in divisive rhetoric and manages only the barest veneer of religiosity. But that was never the issue. Fox evangelicals don’t back Mr. Trump despite their beliefs, but because of them.

Consider the so-called War on Christmas, which the president has made a pet crusade. Mr. Trump has been sharing Christmas greetings since October, well before decorations had even shown up in most stores, when the Values Voter Summit crowd gave him a standing ovation for declaring, “We’re saying ‘Merry Christmas’ again!” He has spent November and December taking victory laps, telling crowds at political rallies in Utah and Florida that “Christmas is back, better and bigger than ever before.”

Every one of Mr. Trump’s predecessors declared “Merry Christmas,” too — including Barack Obama, whose message at last year’s Christmas tree-lighting ceremony was virtually indistinguishable from Mr. Trump’s. What matters to Fox evangelicals, though, is not that Mr. Trump observes Christmas but that he casts himself as the defender of the Christian holiday.