"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."
"In God we trust."
Both these statements are known world-wide and feature one key aspect: the word God. Not just a noun, but a proper noun. A proper noun that specifies this god as God, the Christian deity. This violates the first amendment, regarding the Establishment Clause.
However, if these examples were amended to include a non-specific god, then it would not violate the Establishment Clause, as the god in question could not be identified with any religion.