At Monday's inauguration of his second term, President Barack Obama will raise his right hand and place his left on not one, but two Bibles: One owned by Abraham Lincoln and the other by Martin Luther King Jr.

The Constitution requires he give this oath of office: "I do solemnly swear (or affirm) that I will faithfully execute the office of President of the United States, and will to the best of my ability preserve, protect, and defend the Constitution of the United States."

You might recall that at his 2009 inauguration, President-elect Obama and Chief Justice John Roberts played some kind of "mad libs" with this oath during the swearing-in ceremony, muddling it so badly that they had to redo it a few days later. But why does the president swear on a Bible? Why doesn't he place his hand on the U.S. Constitution -- the very document he's promising to "preserve, protect and defend"?

The Constitution does not require that the president take the oath of office by swearing on a Bible. That would have been a very simple requirement for the constitutional drafters to include. To the contrary, the Founders wanted to ensure that Americans of any faith -- or no faith -- could hold federal office.

They set it forth plainly in Article VI: "... No religious test shall ever be required as a qualification to any office or public trust under the United States."