You'd think that with its mere 36 words, the official presidential oath of office would be pretty straightforward. Here it is, as set out in Article II of the Constitution:
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.
The difficulty comes, however, from what the Constitution doesn't say: "So help me God." So where did those words come from, anyway? Here are five milestones in the contested history of those four words.
1789: Washington's Inauguration
George Washington has long been credited with instituting the tradition of concluding the oath of office with "So help me God," at his inauguration in 1789. The most commonly cited account is this one in Washington Irving's George Washington, A Biography:
The oath was read slowly and distinctly, Washington at the same time laying his hand on the open Bible. When it was concluded, he replied solemnly, “I swear—so help me God!” Mr. Otis would have raised the Bible to his lips, but he bowed down reverently and kissed it.
For more details about this and similar accounts, see this book excerpt.
1853: Pierce's Inauguration
Although Franklin Pierce didn't do much to shake up the "So help me God" tradition, it's worth noting that he is the only president to have chosen to affirm, rather than swear, the oath of office. For a brief explanation of what the difference is and why it matters, see this NPR piece.
1881: Arthur's Inauguration
In a USA Today article casting doubt on the idea that Washington included "So help me God" in his oath, an editor at the U.S. Senate Historical Office stated that the first eyewitness documentation of a president saying "So help me God" was not of Washington's, but of Chester A. Arthur's 1881 inauguration, recorded in the New York Times. One writer noted that the Washington story "may be as apocryphal as the one about the cherry tree." Although Irving was, in fact, an eyewitness of Washington's inauguration, he was six years old at the time, which is why some scholars have given his account less weight in the whole did-Washington-say-it-first debate.
1901: Roosevelt's Inauguration
When Theodore Roosevelt took the oath of office following the death of William McKinley, he omitted "So help me God." According to Theodore Roosevelt: His Life and Work, after he repeated the oath, he concluded simply, "And thus I swear."
2009: Obama's Inauguration
Prior to the inauguration of Barack Obama, the famously litigious atheist Michael Newdow filed a lawsuit seeking to prevent Chief Justice John Roberts from reciting "So help me God" while administering the oath to Obama. (Obama had requested that Roberts include the phrase in the oath.) The Supreme Court rejected hearing his case, stating that he lacked the necessary legal standing. Ultimately, Obama did include "So help me God" in his oath.
Among our more recent presidents, the inclusion of "So help me God" has been the norm. But because the late 1700s were decidedly C-SPAN-free, we'll never know for sure whether George Washington actually started the tradition of adding the words "So help me God" to the end of the constitutional oath of office.
In the National Constitution Center's newest exhibit, Discover the Real George Washington: New Views from Mount Vernon, visitors can witness a recreation of Washington's inauguration day, with a lifelike figure of Washington placing his hand over the Bible. Also in the exhibit, with a replica Bible and pre-recorded oath, you can take the oath office yourself--and choose to include "So help me God" or not.