Like so many presidents before him, Barack Obama has invited a revered guest to his inauguration: God.
Although the Constitution forbids the government from establishing religion, faith is once again figuring prominently into the nation's grandest political pageant, just as it has over the course of American history.
Whether Republican, Democrat or Whig, presidents from the nation's beginnings have invoked the Almighty's powerful hand to convey their visions in times of calamity and contentment alike.
George Washington credited the birth of a young America to "providential agency."
Abraham Lincoln, delivering his second inaugural shortly before the end of the Civil War, said God was punishing a bloodied people for the evils of slavery.
John F. Kennedy called in the middle of the Cold War for heeding the command of the prophet Isaiah "to let the oppressed go free."
In his inaugural address, George H.W. Bush announced, "My first act as president is a prayer. I ask you to bow your heads."
Atheists, agnostics and others have long objected to the use of religious language and symbols in inaugurations and in other functions of government.
A collection of groups and individuals -- led by a California attorney who has filed unsuccessful lawsuits to remove God from the Pledge of Allegiance and money -- sued again to keep "so help me God" out of the inaugural ceremony. (A judge last week refused to grant the request.) The phrase is not mandated by the Constitution but has been uttered repeatedly by presidents, following a precedent some historians believe was set by Washington at his first inauguration April 30, 1789.
Some legal scholars and historians label such "ceremonial deism" constitutionally permissible because it reflects culture and tradition more than religion. As a result, experts believe God will continue his inaugural role, just as he has for more than two centuries.
Washington used "religion in public life to promote national identity and cohesion," to "authenticate public acts" and to communicate fundamental values to an infant country, according to a forthcoming book, "Religion and the American Presidency."
"Presidents have recognized that the American people are overwhelmingly religious," said Gaston Espinosa, the book's editor and an associate professor of religious studies at Claremont McKenna College.
