One Nation Under God. In God We Trust. The first continental Congress began their sessions with Prayer. The leader of our nation, the highest office of the Government of the United States of America has ALWAYS been held by a Man who swore an oath with his hand upon the Holy Bible as an affirmation of sacredness and ended his oath with "So help me God".
The evidence suggests that God exists and that He is quite imminent in his creation. Therefore, God is certainly and currently a "principle" in the US. He is present regardless of laws outlawing His representation, and He is present regardless of whether or not the majority of folks acknowledge His presence. With these facts in mind, doesn't it seem rather foolish to pretend that He doesn't exist in either the private or public square?
Our founding fathers all agreed the United States shouldn't be run by a religion, or favor one religion over the other which is why they put in the establishment clause. The very first sentence of the First Amendment of the Constitution says Congress may not establish any religion of the United States which means no religion will be endorsed, favored, or allowed in our political system.
This country was founded by some of the wisest men of their time. One trait that was common amongst them was their tendency to lean towards secularism. To quote Thomas Jefferson, author of the United States' Declaration of Independence,
"In every country and every age, the priest has been hostile to liberty. He is always in alliance with the despot ... They have perverted the purest religion ever preached to man into mystery and jargon, unintelligible to all mankind, and therefore the safer engine for their purpose."
Clearly, Jefferson saw the corruption that comes with religion when it is used as a principle in government. It wasn't just Jefferson either, Benjamin Franklin, Thomas Paine, John Adams, James Madison, and even George Washington are all known for their secularist beliefs and their desire to keep it as far away from politics that they could. Now, in modern day society, we have phrases like, "In God We Trust" on our money. What a lot of people are unaware of is the fact that this did not start appearing on our paper money until 1956 after the second "Red Scare" when America was terrified of communism. The idea that this country was founded on Christianity (and I am going to assume that by "God" we mean the Christian God in this case) is absurd. It is an idea that is based on personal beliefs, rather than actual facts.
Although many Americans believe in God, religion is not a part of the American government system according to the constitution. There is officially a separation between church and state. People are free to believe in whatever god they want, but they cannot use these beliefs to influence political or government related affairs.