On Monday the United States will celebrate one of its great festivals of civil religion as Barack Obama is inaugurated for a second time. Although nothing in the Constitution mandates it (the only things the Constitution specifies are the date and the wording of the oath), the ceremony will include an invocation, a benediction, undoubtedly one or more mentions of God in the inaugural address, and the words “so help me God” as part of the oath of office. These words are often attributed to George Washington (he allegedly added them to his oath in 1789, but no extant contemporary evidence proves that he did). The historical record indicates instead that these words were probably first spoken in 1881 by Chester Arthur when he was sworn in following James...