The First Amendment to the United States Constitution, at its very outset, states: "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof."
Sixteen little words that say it all: the Establishment clause, and the Free Exercise clause.
Thomas Jefferson gave an interpretation of the First Amendment, in his January 1st, 1802 letter to the Committee of the Danbury Baptist Association, calling it a “Wall of Separation between Church and State.”
James Madison also wrote that “Strongly guarded… is the separation between religion and government in the Constitution of the United States.”
What more need be said?