Why shouldn't  Barack Obama say "so help me God" when he takes the oath of office on Jan. 20? That's what I wondered when I learned of the lawsuit, to be heard by a federal judge this week, which aims to prevent him from doing so. Obama is a religious man who has spoken often and eloquently about God. It seemed to me reasonable that he would want to ask for divine guidance on this, the most momentous day of his life—at such a critical time for our country and the world.

On the right, the lawsuit—filed by the atheist gadfly Michael Newdow—is being derided as frivolous. Even some of Newdow's ideological allies are steering clear. Barry Lynn, executive director of Americans United for the Separation of Church and State, says the suit is ill timed and would not have joined it, if asked. "I don't believe the judicial climate in 2009 is likely to generate success for this lawsuit," he says.

On close inspection, though, Newdow's case offers an important lesson in American history and public piety. Many Americans expect and even desire a certain amount of religious pomp at presidential Inaugurations because each ceremony in recent memory—captured on television—has had plenty of it. We believe presidents always say "so help me God" as they're being sworn in. They always invite one or two (sometimes three or four) clerics to offer public prayers before and after the ceremony—another element that Newdow wants excised from the proceedings.

Except they don't. The congressional oath, which is used for members of Congress and other government employees, includes the phrase "so help me God." The presidential oath of office, laid down in the Constitution, does not. The incoming president closes his oath by swearing to "preserve, protect and defend the Constitution of the United States." That's it. The fact is, according to Donald Ritchie, a historian at the Senate Historical Office, we have no idea what most 19th-century presidents have said about God as they were sworn in because for most of American history there were no microphones and no recording devices. Legend has it that George Washington said "So help me God" at his Inauguration, but new scholarship shows that this story may be as apocryphal as the one about the cherry tree.

Similarly, prayers offered by guest clerics—which this year have stirred such controversy—are a 20th-century phenomenon. Ritchie says the first known instance of an invocation by an outsider was probably in 1941, when Franklin D. Roosevelt, an Episcopalian looking ahead to war, invited a Catholic priest to pray. As the Cold War began with its threat of what was known as "Godless communism," American expressions of religiosity grew fervent. "Under God" in the Pledge of Allegiance, "In God We Trust" as the national motto—all these were innovations of the 1950s. In 1957, Dwight D. Eisenhower invited four clerics, including a rabbi and a Greek Orthodox archbishop, to pray at his Inauguration. Billy Graham offered prayers at eight Inaugurations starting in 1953, his angular face an international symbol of American Christian piety.

What we think of, then, as the conventional religiosity of Inaugurations is conventional only by recent standards—and conventions, as Obama well knows, can change. According to an affidavit by Jeffrey Minear, chief of staff to Chief Justice John Roberts, who is the defendant in the lawsuit, Obama wants to say "so help me God"—and by all means, he should do so. The public prayers by two Christian ministers are more problematic. Today, the greatest threats to our safety come not from godless communists but from religious fundamentalists abroad. Our new president might use his Inauguration then to showcase the values that have made this country great: pluralism, moderation—and the separation of church and state. Though not as politically expedient, the better choice might be to pray in private.

© 2009