What stuns me is how my fellow evangelicals can rally behind a man whose words and actions are so at odds with the central teachings of our faith. They overlook, rationalize and even delight in Mr. Trump’s obsessive name-calling and Twitter attacks, his threats and acts of intimidation, his vindictiveness and casual cruelty (including mocking the disabled and P.O.W.s), all of which masquerade as strength and toughness. For some evangelicals, Christianity is no longer shaping their politics; with Mr. Trump in view, their faith lies subordinate.

Yet it goes beyond that. Trumpism is not a political philosophy; it is a purposeful effort, led by a demagogue, to incite ugly passions, stoke resentments and divisions, and create fear of those who are not like “us” — Mexicans, Muslims and Syrian refugees. But it will not end there. There will always be fresh targets.

Mr. Trump’s approach to life is not new. In “The Republic,” Plato writes of Thrasymachus debating Socrates over the meaning of justice. Thrasymachus, a cynical Sophist, insists that justice has no intrinsic meaning but is merely a pretty word for what is in the interest of the stronger party. Life is a competition to get more money and more power; that is what defines success. “Injustice, if it is on a large enough scale, is stronger, freer, and more masterly than justice,” he argues.

Almost four centuries later, a carpenter from Nazareth offered a very different philosophy. When you see a wounded traveler on the road to Jericho, Jesus taught, you should not pass him by. “Truly I say to you,” he said in Matthew, “to the extent that you did it to one of these brothers of mine, even the least of them, you did it to me.”

At its core, Christianity teaches that everyone, no matter at what station or in what season in life, has inherent dignity and worth. “Follow justice and justice alone,” Deuteronomy says, “so that you may live and possess the land the Lord your God is giving you.” The attitude of Thrasymachus is foreign to biblical Christianity. So is Trumpism. In embracing it, evangelical Christians are doing incalculable damage to their witness.