Religious nationalism is now out of favor for many Americans, but as recently as 1942 you could hear Franklin Roosevelt justify America’s involvement in World War II with biblical rhetoric. In his State of the Union address that year, Roosevelt declared not only that the world was too small for “Hitler and God” but that the United States was fighting “inspired by a faith that goes back through all the years to the first chapter of the Book of Genesis.” The idea that “God created man in His own image,” he said, was the reason to fight and “uphold the doctrine that all men are equal in the sight of God.”

Of...