The First Amendment states that there is a separation of Church and State, therefore the Government has no religion. However, a majority of people in this country practice Christianity, which is said to be a monotheistic religion, although Catholicism seems to defy monotheism with it's worship of the many saints and Mary.
The United States has no national religion. No religion is allowed to be endorsed by the state, so there is no way that claims that the US is a monotheistic state are valid. The US is not an anything state. Although people like to pretend otherwise, the United States is supposed to be secular for the most part, in order to stop the prosecution of minority religious groups.