"In God We Trust" was a phrase that was added to quarters in 1908, during a resurgence of the power of the religious right in the United States. Within 10 years, it was added to the majority of other coins, and subsequently to paper bills as well. Earlier, during the late 1800s, fundamentalist politicians tried to pass a resolution allowing the modification of the Constitution to include clauses indicating that the United States was a nation "Under God" or "Under Almighty." This legislation, thankfully, was not allowed to pass and deemed Unconstitutional.
Later, in 1954, the phrase "Under God" was added to the Pledge of Allegiance (written by a socialist in the 19th Century) to help unite the country against the intimidating communist threat. The Religious Right believed that, simply, we needed to reaffirm our religiosity in the face of the communists who believed, as Karl Marx so tactfully put it, "Religion is the Opiate of the Masses." From the Communist Manifesto, this phrase was everything that the politicians in power in the US during this time were striving to refute.
What was overlooked at this time (and is still overlooked now) is the overwhelmingly non-religious views of our Founding Fathers. A quote from Thomas Paine: "The whole religious complexion of the modern world is due to the absence from Jerusalem of a lunatic asylum." Granted, Thomas Paine was an Anarchist, but he was also vital to the success of the American Revolution with the publication of his pamphlet Common Sense.
Since the founding of the modern United States (this was following the signing of the Articles of Confederation), there has been a single clause that has granted to the United States Government success in the world economy and community. The prospect of "Separation of Church and State" is the most important concept in the US constitution, for it keeps religious influence outside the walls of the US government. Throughout this entire conflict that America is involved in now regarding the phrase "Under God," one must ask oneself the original intention of those who wrote the Pledge of Allegiance, and the original intention of those who wrote the Constitution of the United States of America. Only then, after sufficiently considering this and its implications, can one make a judgment as to whether or not this phrase, or any phrase including a reference to God, or Allah, or Satan, or even No God, should be included in any federally sanctioned document or legislation.