(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.

Click through the slideshow to see most and least religious cities in the United States:

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  • Salt Lake City, UT

    73,487 religious adherents per 100,000 persons.

  • Birmingham, AL

    Metro area Birmingham-Hoover, AL<br> 70,990 religious adherents per 100,000 persons. <br>

  • Oklahoma City, OK

    60,249 religious adherents per 100,000 persons.

  • Memphis, TN-MS-AR

    Metro area: Memphis, TN-MS-AR<br> 58,847 religious adherents per 100,000 persons. <br>

  • Pittsburgh, PA

    57,695 religious adherents per 100,000 persons.

  • Chicago, IL-IN-WI

    Metro area: Chicago-Joliet-Naperville, IL-IN-WI<br> 57,162 religious adherents per 100,000 persons. <br>

  • Providence, RI-MA

    Metro area: Providence-New Bedford-Fall River, RI-MA<br> 56,840 religious adherents per 100,000 persons. <br>

  • Boston, MA-NH

    Metro area: Boston-Cambridge-Quincy, MA-NH<br> 56,765 religious adherents per 100,000 persons.<br>

  • New York, NY-NJ-PA

    Metro area: New York-Northern New Jersey-Long Island, NY-NJ-PA<br> 55,595 religious adherents per 100,000 persons. <br>

  • Dallas, TX

    Metro area: Dallas-Fort Worth-Arlington, TX<br> 55,536 religious adherents per 100,000 persons. <br>

  • Houston, TX

    Metro area: Houston-Sugar Land-Baytown, TX<br> 55,144 religious adherents per 100,000 persons. <br>

  • Buffalo, NY

    Meto area: Buffalo-Niagara Falls, NY<br> 54,706 religious adherents per 100,000 persons. <br>

  • Philadelphia, PA-NJ-DE-MD

    Metro area: Philadelphia-Camden-Wilmington, PA-NJ-DE-MD<br> 54,668 religious adherents per 100,000 persons. <br> Credit: Wikimedia Commons. Original photo <a href="http://upload.wikimedia.org/wikipedia/commons/3/39/Independence_Hall.jpg" target="_hplink">here</a>.

  • San Antonio, TX

    Metro area: San Antonio-New Braunfels, TX<br> 54,093 religious adherents per 100,000 persons. <br>

  • New Orleans, LA

    Metro area: New Orleans-Metairie-Kenner, LA<br> 53,479 religious adherents per 100,000 persons. <br>

  • Minneapolis, MN-WI

    Metro area: Minneapolis-St. Paul-Bloomington, MN-WI<br> 52,061 religious adherents per 100,000 persons. <br>

  • Milwaukee, WI

    Metro area: Milwaukee-Waukesha-West Alis, WI<br> 51,823 religious adherents per 100,000 persons. <br>

  • Charlotte, NC-SC

    Metro area: Charlotte-Gastonia-Rock Hill, NC-SC<br> 51,764 religious adherents per 100,000 persons. <br>

  • Los Angeles, CA

    Los Angeles-Long Beach-Santa Ana, CA<br> 51,382 religious adherents per 100,000 persons. <br>

  • Cleveland, OH

    Metro area: Cleveland-Elyria-Mentor, OH<br> 51,185 religious adherents per 100,000 persons. <br>

  • Nashville, TN

    Metro area: Nashville-Davidson-Murfreesboro-Franklin, TN<br> 50,694 religious adherents per 100,000 persons. <br>

  • Louisville, KY-IN

    Metro area: Louisville-Jefferson County, KY-IN<br> 50,445 religious adherents per 100,000 persons. <br>

  • Richmond, VA

    50,113 religious adherents per 100,000 persons.

  • Jacksonville, Florida

    50,093 religious adherents per 100,000 persons.

  • Atlanta, GA

    Metro area: Atlanta-Sandy Springs-Marietta, GA<br> 49,721 religious adherents per 100,000 persons. <br>

  • Kansas City, MO-KS

    49,702 religious adherents per 100,000 persons.

  • St. Louis, MO-IL

    48,889 religious adherents per 100,000 persons.

  • Hartford, CT

    Metro area: Hartford-West Hartford-East Hartford, CT<br> 47,226 religious adherents per 100,000 persons. <br>

  • Cincinnati, OH-KY-IN

    Metro area: Cincinnati-Middletown, OH-KY-IN<br> 44,672 religious adherents per 100,000 persons. <br>

  • Detroit, MI

    Metro area: Detroit-Warren-Livonia, MI<br> 44,583 religious adherents per 100,000 persons. <br>

  • Washington, DC-VA-MD-WV

    Metro area: Washington-Arlington-Alexandria, DC-VA-MD-WV<br> 44,576 religious adherents per 100,000 persons. <br>

  • Raleigh, NC

    Metro area: Raleigh-Cary, NC<br> 44,469 religious adherents per 100,000 persons. <br>

  • San Jose, CA

    Metro area: San Jose-Sunnyvale-Santa Clara, CA<br> 44,189 religious adherents per 100,000 persons. <br>

  • Rochester, NY

    44,066 religious adherents per 100,000 persons.

  • Austin, TX

    Metro area: Austin-Round Rock-San Marcos, TX<br> 43,921 religious adherents per 100,000 persons. <br>

  • San Diego, CA

    Metro area: San Diego-Carlsbad-San Marcos, CA<br> 43,917 religious adherents per 100,000 persons. <br>

  • Riverside, CA

    Metro area: Riverside-San Bernardino-Ontario, CA<br> 42,293 religious adherents per 100,000 persons. <br>

  • Indianapolis, IN

    Metro area: Indianapolis-Carmel, IN<br> 42,213 religious adherents per 100,000 persons. <br>

  • Baltimore, MD

    Metro area: Baltimore-Towson, MD<br> 42,077 religious adherents per 100,000 persons. <br>

  • Virginia Beach, VA-NC

    Metro area: Virginia Beach-Norfolk-Newport News, VA-NC<br> 40,417 religious adherents per 100,000 persons. <br>

  • Orlando, FL

    Metro area: Orlando-Kissimmee-Sanford, FL<br> 40,631 religious adherents per 100,000 persons. <br>

  • Miami, FL

    Metro area: Miami-Fort Lauderdale-Pompano Beach, FL<br> 38,058 religious adherents per 100,000 persons. <br>

  • San Francisco, CA

    Metro area: San Francisco-Oakland-Fremont, CA<br> 37,796 religious adherents per 100,000 persons. <br>

  • Columbus, OH

    37,702 religious adherents per 100,000 persons.

  • Denver, CO

    Metro area: Denver-Aurora-Broomfield, CO<br> 36,968 religious adherents per 100,000 persons. <br>

  • Phoenix, AZ

    Metro area: Phoenix-Mesa-Glendale, AZ<br> 37,518 religious adherents per 100,000 persons. <br>

  • Sacramento, CA

    Metro area: Sacramento-Arden-Arcade-Rosenville, CA<br> 36,512 religious adherents per 100,000 persons. <br>

  • Las Vegas, NV

    Metro area: Las Vegas-Paradise, NV<br> 35,761 religious adherents per 100,000 persons. <br>

  • Seattle, WA

    Metro area: Seattle-Tacoma-Bellevue, WA<br> 35,643 religious adherents per 100,000 persons. <br>

  • Tampa, FL

    Metro area: Tampa-St. Petersburg-Clearwater, FL<br> 34,761 religious adherents per 100,000 persons. <br>

  • Portland, OR-WA

    Metro area: Portland-Vancouver-Hillsboro, OR-WA<br> 31,591 religious adherents per 100,000 persons. <br>

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