This is probably an extreme example of one of the conceits of the religious right, which is that the United States was founded by Christians for Christians, and that our laws and policies should be based on the Christian faith. However, like many beliefs of the religious right, it is based purely on wishful thinking with no basis in historical fact.
It's true that "In God we Trust" is the official motto of the United States, but that was only adopted in 1956. The phrase did not appear on any money until 1864.
The Pledge of Allegiance as it was composed in 1892 and adopted by Congress in 1942 did not contain the phrase "under God." That phrase was added by Congress in 1954.
Most significant is that in 1797, just seven years after the Constitution of the United States was ratified, the Senate unanimously ratifed, and President John Adams signed, a treaty with Tripoli which contained the following Article 11:
Art. 11. As the Government of the United States of America is not, in any sense, founded on the Christian religion,—as it has in itself no character of enmity against the laws, religion, or tranquility, of Mussulmen [Muslims],—and as the said States never entered into any war or act of hostility against any Mahometan [Muslim] nation, it is declared by the parties that no pretext arising from religious opinions shall ever produce an interruption of the harmony existing between the two countries.The apparent purpose of this article 11 was to assure Muslim nations that United States had no intention of engaging in any religious wars and that there was no reason why the United States would not peacefully co-exist with Muslim nations.
Which is quite a contrast to the rhetoric that is often heard from the right, many of whom might be described as Islamaphobic.