Pledge of Allegiance
Originally written in 1898 by Frances Bellamy the current United States pledge reads:
"I pledge allegiance to the flag of the United States of America, and to the Republic for which it stands: one Nation under God, indivisible, with Liberty and Justice for all."
The pledge of Allegiance is relevant in that in 1954 the words "under God" were added. This is often referenced by apologists as evidence that America is a Christian nation.
9th Circuit decision in Newdow v. Elk Ridge
To recite the pledge is not to describe the United States; instead it is to swear allegiance to the values for which the flag stands: unity, indivisibility, liberty, justice and -- since 1954 -- monotheism