"These inaugurals and religious language are used to bring the nation together and to show that God is looking over the people.
"It's an affirmation of the role religion plays in American polity, but it's also a way of respecting the boundaries that the Founders set," Espinosa added. "They wanted to avoid the extremes of church and state being blended . . . and of trying to eradicate the church from all discourse."
Religion will infuse this week's inaugural festivities.
Aside from the oath, Obama has invited ministers to deliver prayers before, during and after the inauguration. The openly gay Episcopal bishop from New Hampshire, V. Gene Robinson, delivered the invocation for the opening inaugural event Sunday at the Lincoln Memorial.
The Rev. Rick Warren of Orange County's Saddleback Church, one of the largest evangelical congregations in the country, will give the inaugural invocation. The Rev. Joseph E. Lowery, a civil rights leader, will deliver the benediction.
On Wednesday, the Rev. Sharon E. Watkins, the general minister and president of the Christian Church (Disciples of Christ) will give the sermon at the National Prayer Service in Washington, D.C.'s National Cathedral.
Atheists, humanists and other groups argue that the inauguration's many religious features violate the 1st Amendment's ban against government establishment of religion.
Eleven groups and 30 individuals sued Chief Justice John G. Roberts Jr., Warren, Lowery and others involved in the inauguration to strip the "God" language from the oath and stop the inaugural prayers. The groups also object to the president-elect's plan, like those before him, to swear the oath on a Bible.
Lead plaintiff Michael Newdow, a Sacramento attorney, pointed out that the Constitution requires the incoming president only to swear that he "will faithfully execute the office of president" and defend the Constitution. It does not require the use of a Bible.
"The government is not supposed to be taking sides," Newdow said. "When we use 'so help me God,' we are saying that those who believe in God are better than those who do not believe in God. This is about people who want the government to treat every religious view equally."
Also not required is a morning worship service, a custom observed almost every term since Franklin D. Roosevelt visited St. John's Episcopal Church in Washington before his 1933 inauguration.
