When George Washington took the Oath of Office on April 30, 1789,
as mandatory proscribed by the Constitution (Article II, Secion 1,
Clause 8), he said:

"I, George Washington, do solomnly swear that I will
faithfully execute the Office of the President of the
United States, and will to the best of my ability, preserve,
protect and defend the Constitution of the United States."

Then, he added, "So help me God."

It is believed that every President since has added, "So help me God."

From page 188, Always Growing in the Faith by Gene Dirr.
Available at www.amazon.com .