At the Values Voter Summit in Washington DC last weekend, leading members of the American religious right rallied around the notion that the US is a Christian nation. One speaker, the leader of a Texas megachurch, cited the advice of John Jay, a revolutionary-era American statesmen, that Americans should elect Christians. In an 1816 letter, Jay wrote: "Providence has given to our people the choice of their rulers, and it is the duty, as well as the privilege and interest, of our Christian nation to select and prefer Christians for their rulers." When Jay and other 18th and 19th-century Anglo-American evangelicals spoke of "Christian", they meant Protestant. Catholics, Jay was implying, are not Christians, as well as suspect Americans.
Then as now, speaking of the US as Christian nation remains as much a negative as a positive act, though the excluded has changed from Catholics to Muslims. Despite the anti-Islamists' claims of endemic conflict, their particular cause is a new and superficial characteristic of American political culture. The relevant history is not in any inherent conflict with Islam, but in the need of American political culture for a foreign devil. Not so long ago, atheistic communists played that role. For most of American history, however, the foreign devil was Catholicism.
Anti-Catholicism has deep roots in American political culture. A trace of its one-time prominence remains evident in the oath of allegiance for US citizenship. The oath of Allegiance requires prospective US citizens to "renounce and abjure all allegiance and fidelity to any foreign prince, potentate, state, or sovereignty". The "potentate" was the Pope.
Jay and other Americans of the revolutionary era cared little about Islam, and knew even less. On the other hand, anti-Catholicism enjoyed a vibrant career at all levels of American society. It was Jay, for example, who wrote the Continental Congress's 1774 address to the people of Great Britain. The address alleged that, with the Quebec Act, the British parliament's policy of religious toleration for Quebec Catholics advanced a plot against America. Recognising the right of Catholics to religious freedom in North America, Jay wrote, would encourage "swelling" waves of Catholic immigrants from Europe. It would soon "reduce the ancient free Protestant colonies" to a "state of slavery."
http://www.guardian.co.uk/commentisfree/belief/2011/oct/15/catholicism-america-forign-devil