9nap, our U.S. currency does not have "One nation under God" written on it. It does have "In God We Trust".
"Separation of church and state" is a phrase used by Thomas Jefferson and others expressing an understanding of the intent and function of the Establishment Clause and Free Exercise Clause of the First Amendment to the Constitution of the United States which reads: "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof..."
The phrase "separation of church and state" is generally traced to a January 1, 1802 letter by Thomas Jefferson, addressed to the Danbury Baptist Association in Connecticut, and published in a Massachusetts newspaper.