What is the official religion in the United States?

The United States does not have an official religion.

First Amendment of the US Constitution:

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.

An excerpt from Thomas Jefferson's "Danbury Letter" of 1802, about the meaning of the First Amendment:

"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 prohibit the free exercise thereof, thus building a wall of separation between between church and state."

Article VI of the U.S. Constitution:

"... no religious Test shall ever be required as a Qualification to any Office or public Trust under the United States."

The site below includes comments by James Madison on this subject. It might be noted that arguments over the teaching of religion in U.S. schools, for example, are hardly new. They go back well over 100 years:

http://www.heritage.org/constitution/#!/articles/6/essays/135/religious-test

http://www.loc.gov/exhibits/religion/rel06-2.html

Despite Jefferson's Danbury Letter indicating a separation of church and state, leading to an indication that Jefferson did not want the church to play any hand in the Federal Government, Jefferson was still an avid church goer to services that were held in the Capitol Building in Washington, DC.

In his diary, Manasseh Cutler (1742-1823), a Federalist Congressman from Massachusetts and Congregational minister, notes that on Sunday, January 3, 1802, John Leland preached a sermon on the text "Behold a greater than Solomon is here. Jef[ferso]n was present."

Thomas Jefferson attended this church service in Congress, just two days after issuing the Danbury Baptist letter. Leland, a celebrated Baptist minister, had moved from Orange County, Virginia, and was serving a congregation in Cheshire, Massachusetts, from which he had delivered to Jefferson a gift of a "mammoth cheese," weighing 1235 pounds.

Private behaviors and proclivities do not necessarily reflect what a politician believes a government should be doing insofar as religion is concerned. Both Jefferson and Madison firmly believed that the government should stay out of religious affairs.

It should be noted that Jefferson's conception of Christianity was, in any case, unorthodox:


http://www.therealpresence.org/archives/Sacred_Scripture/Sacred_Scripture_013.htm

The US is unique in the Western world for not having an official religion. It is still quite a radical notion in Europe. Even Ireland, which was one of the first European countries to adopt the notion of the separation of Church and State, still bases the need for such a separation on biblical quotes, and most of its citizens have their religion in common.

In fact, neither the Irish nor the Americans were the first to think up separation of religious and official matters. The book of Deuteronomy stresses a need for separation of powers, from the king who is in charge of political matters to the high priest who controled religious matters to the courts which were placed in charge of civil matters, economics, and some domestic policies.