During the Christmas season New York City -- a bastion of radical secularism -- had a beautiful Nativity scene set up right in Central Park. It met every legal standard under the free exercise clause because it was set up by a private organization (the Catholic League for Religious and Civil Rights, I believe) with all of the required city permits.
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.