Our constitution's first ammendment says:"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."
This simply means that there can be no official church or religion of the United States such as the Church of England. It's important to note the "or prohibiting the free exercise thereof" part because that's what so many people would like to do to all elected officials.