Another answer included the following:
The majority religion is Roman Catholic. Have a look:
Just to clarify, that map (on my blog), only shows numbers of counties dominated by a given religion. Indeed, the number of Catholic counties compared with the number of Baptist Convention counties is nearly identical, with Catholic only slightly exceeding. But, again, it doesn't show the populations of individuals in each religion.
With regard to whether the USA is a Christian Nation.
That would depend upon the definition of "Christian Nation." If one considers the majority religion in the country to be a defining factor, then one must concede that the USA *is* a Christian Nation.
If, however, one considers the nation as a reflection of ideals, principles, and essence of the American nation as conceived by the Founding Fathers, then it is definitely *not* a "Christian Nation," but a nation founded on freedom and the pursuit of happiness regardless of religious belief or non-belief.
Many of the most significant of our Founding Fathers were not simply indifferent to Christianity in order to be fair or objective, they expressed an opposition to Christianity.
Thomas Jefferson was quoted as saying in a letter to the Danbury Baptists, "Believing with you that religion is a matter which lies solely between Man & his God, that he owes account to none other for his faith or his worship, that the legitimate powers of government reach actions only, & not opinions, I contemplate with sovereign reverence that act of the whole American people which declared that their legislature should "make no law respecting an establishment of religion, or prohibiting the free exercise thereof," thus building a wall of separation between Church & State. Adhering to this expression of the supreme will of the nation in behalf of the rights of conscience, I shall see with sincere satisfaction the progress of those sentiments which tend to restore to man all his natural rights, convinced he has no natural right in opposition to his social duties ."
Neither God nor Christianity is mentioned in the Constitution  and the only place where "religion" is mentioned is to prohibit "religious tests" for office.
In 1797, the Treaty of Tripoli was signed , which included the line, "... the Government of the United States is not, in any sense, founded on the Christian religion;" This treaty process was begun by George Washington and concluded during the administration of John Adams, the second U.S. President.
The United States is not a Christian nation any more than it is a Neo-Conservative nation or a Liberal one; no more than it is a baseball, football, or a basketball nation; nor more than it is a Coka Cola or a Pepsi nation. It is, of course all these things, but anyone is free to side with any, all or none.
1. Jefferson, Thomas (1802, Jan 1). Jefferson's Letter to the Danbury Baptists. Library of Congress, found online at: http://www.loc.gov/loc/lcib/9806/danpre.html
2. United States (1787). The United States Constitution. Found online at: http://en.wikipedia.org/wiki/United_States_Constitution
3. United States (1797). The Treaty of Tripoli. Library of Congress. Found online at: http://memory.loc.gov/cgi-bin/ampage?collId=llsp&fileName=002/llsp002.db&Page=18