The United States is, in theory, a secular nation. Despite the occasional verbal hat tips to a supernatural watchmaker by some of the more deistic leaning founders, all of America’s founding documents are secular: they embrace no official state religion of any kind and maintain a strict separation between church and state. This political structure has, in turn, made the U.S. one of the most religiously pluralistic societies in the world. After all, having freedom of religion ensures that all religions can be practiced openly.
In practical terms, however, for much of its history the U.S. has been a majority Christian Protestant nation. The first European settlers (with the exception of some pesky Spanish Catholics in Florida and out west) to America were Protestants, and a Protestant religious tradition has shaped much of American history. And, of course, the violent, sectarian brouhaha that is Christian history ensured that a predominantly Protestant United States would also have its fair share of Anti-Catholic sentiment.