(RNS) When President Obama rests his hand on two historic Bibles to take his second-term oath of office Monday (Jan. 21), he’ll add a phrase not mentioned in the Constitution: “So help me God.”

But the Almighty’s role on the Capitol steps is a controversial one.

First, there was a myth that the tradition of adding God to the oath began with George Washington. It didn’t, say experts at the Library of Congress, the U.S. Senate Historical Office and the first president’s home, Mount Vernon.

Although the phrase was used in federal courtrooms since 1789, the first proof it was used in a presidential oath of office came with Chester Arthur’s inauguration in September 1881.

Every president since, including Obama, has followed suit.

California atheist activist Michael Newdow has battled unsuccessfully in federal court to ban the phrase. Obama notified Chief Justice John Roberts, who administers the oath, that he wanted this phrase included. In 2011, the U.S. Supreme Court declined to hear Newdow’s last appeal.

Four years later, “so help me God” is unchallenged. Obama will once more use the Bible that Abraham Lincoln used in 1861. This term, he’ll add a second Good Book, the Bible that Martin Luther King Jr. carried with him in all his travels.

Myrlie Evers-Williams, past chairman of the NAACP and widow of murdered civil rights leader Medgar Evers, will give the invocation. The first woman to offer the invocation, she’s not a member of the clergy.

The benediction will be offered by the Rev. Luis Leon, the rector of St. John’s Church, an Episcopal congregation that sits across Lafayette Square from the White House. A conservative evangelical pastor, the Rev. Louie Giglio from Atlanta, was dumped from the program over his views opposing same-sex marriage.

The inauguration has included blessings by clergy for two centuries. Originally, they were offered by the Senate chaplain. After 1933, the president-elect began naming his choices.

In between the first and the final blessing, there’s always a lot of God talk in the president’s address.

“I challenge you to find any presidential speech that doesn’t make a lot of mention of God,” says constitutional historian R.B. Bernstein who teaches law at New York Law School and political science and history at City College of New York.

George Washington arrived at his inauguration to the sound of church bells. His speech began with “fervent supplications” to the “Almighty Being” and concluded by seeing God’s “divine blessing” for the nation.

Obama’s first inaugural speech called out to Christians, Muslims, Jews, Hindus and, for the first time in any inaugural address, to nonbelievers as well. That’s a wide sweep now that one in five Americans say they have no religious identity.

Tuesday, the focus will shift to prayer for the president with the traditional National Prayer Service at the Washington National Cathedral. The Rev. Adam Hamilton, a United Methodist pastor from Kansas, will preach the sermon.