The Establishment Clause of the First Amendment of the Constitution not only provides for freedom OF religion, but freedom FROM religion.
Putting the phrase "In God We Trust" on US currency not only violates the establishment clause by establishing US government support for the idea of the existence of deity, but establishes a form of government religions acknowledging "trust" in that deity.
More and more Americans do not believe in the God of Abraham...these include Hindus, Buddhists, Pagans, Wiccans, Agnostics, Atheists, Humanists, Naturists and more. Pew estimates that this number is now greater than 40%.
The inclusion of this phrase is divisive and unnecessary - especially when we have a perfectly good motto that is inclusive: E pluribus unum.