Argh. No, it bloody well is not; it was explicitly created as a secular country. Remember that whole "freedom of religion" thing in our Bill of Rights? Remember the whole "in no sense a Christian nation" thing in the Treaty of Tripoli?
US Constitution: Article VI - Debts, Supremacy, Oaths
The Senators and Representatives before mentioned, and the Members of the several State Legislatures, and all executive and judicial Officers, both of the United States and of the several States, shall be bound by Oath or Affirmation, to support this Constitution; but no religious Test shall ever be required as a Qualification to any Office or public Trust under the United States.
US Constitution: Amendment 1 - Freedom of Religion, Press, Expression.
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
Treaty of Peace and Friendship between the United States and the Bey and Subjects of Tripoli of Barbary. by the United States Government
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 tranquillity, of Mussulmen; and, as the said States never entered into any war, or act of hostility against any Mahometan 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.