The United States added "In God We Trust" to our currency and added "under God" to the Pledge of Allegiance in the 1950s.
The argument has been made that this was a violation of the Establishment Clause which states, "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof" and that this act gave special treatment to Christianity.
It was not a violation of the Establishment Clause because it doesn't do anything to help or hinder the establishment of ANY religion, nor does it prohibit the free exercise thereof. It doesn't give overt or undue or inappropriate respect to Christianity.
Many religions refer to their deity as "God"; the phrases are inclusive of many religions, not exclusive.