Donald J. Trump stands astride the polls in the Republican presidential race, beating all comers in virtually every demographic of the primary electorate. Most illogical is his support from evangelicals and other social conservatives. To back Mr. Trump, these voters must repudiate everything they believe.

Ben Carson recently contrasted his own faith in God with Mr. Trump's theatrical egocentrism. "By humility and the fear of the Lord are riches and honor and life, and that's a very big part of who I am," he said, citing a Bible verse. "I don't get that impression with him." Mr. Trump hit back, suggesting that Mr. Carson was faking his own faith: "So I don't know about Ben Carson's faith, and all of a sudden he becomes this great religious figure. I don't think he's a great religious figure." Mr. Carson quickly backed off from his comments, but the questions are not so easily dismissed.

There's no religious test for office, and there shouldn't be. My Baptist ancestors were willing to make alliances with the heretical Thomas Jefferson because he believed in religious liberty. It didn't matter that they never would have let him teach Sunday school.

We should not demand to see the long-form certificate for Mr. Trump's second birth. We should, though, ask about his personal character and fitness for office. His personal morality is clear, not because of tabloid exposes but because of his own boasts. His attitude toward women is that of a Bronze Age warlord. He tells us in one of his books that he revels in the fact that he gets to sleep with some of the "top women in the world." He has divorced two wives (so far) for other women.

This should not be surprising to social conservatives in a culture shaped by pornographic understandings of the meaning of love and sex. What is surprising is that some self-identified evangelicals are telling pollsters they're for Mr. Trump. Worse, some social conservative leaders are praising Mr. Trump for "telling it like it is."

In the 1990s, some of these social conservatives argued that "If Bill Clinton's wife can't trust him, neither can we." If character matters, character matters. Today's evangelicals should ask, "Whatever happened to our commitment to 'traditional family values'?"

Mr. Trump tells us "nothing beats the Bible," and once said to an audience that he knows how Billy Graham feels. He says of evangelicals: "I love them. They love me." And yet, he regularly ridicules evangelicals, with almost as much glee as he does Hispanics. This goes beyond his trivialization of communion with his recent comments about "my little cracker" as a way to ask forgiveness. In recent years, he has suggested that evangelical missionaries not be treated in the United States for Ebola, since they chose to go overseas in the first place.

Still, the problem is not just Mr. Trump's personal lack of a moral compass. He is, after all, a casino and real estate mogul who has built his career off gambling, a moral vice and an economic swindle that oppresses the poorest and most desperate. When Mr. Trump's casinos fail, he can simply file bankruptcy and move on. The lives and families destroyed by the casino industry cannot move on so easily.

He's defended, up until very recent years, abortion, and speaks even now of the "good things" done by Planned Parenthood. In a time when racial tensions run high across the country, Mr. Trump incites division, with slurs against Hispanic immigrants and with protectionist jargon that preys on turning economic insecurity into ugly "us versus them" identity politics. When evangelicals should be leading the way on racial reconciliation, as the Bible tells us to, are we really ready to trade unity with our black and brown brothers and sisters for this angry politician?

Jesus taught his disciples to "count the cost" of following him. We should know, he said, where we're going and what we're leaving behind. We should also count the cost of following Donald Trump. To do so would mean that we've decided to join the other side of the culture war, that image and celebrity and money and power and social Darwinist "winning" trump the conservation of moral principles and a just society. We ought to listen, to get past the boisterous confidence and the television lights and the waving arms and hear just whose speech we're applauding.

Russell Moore is president of the Ethics and Religious Liberty Commission of the Southern Baptist Convention, the nation's largest Protestant denomination. He is the author of "Onward: Engaging the Culture Without Losing the Gospel."