Article VI, clause 3:
No religious Test shall ever be required as a Qualification to any Office or public Trust under the United States.
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.
The First Amendment's Religion Clauses ... embody an idea that was once considered radical: Free people are entitled to free and diverse thoughts, which government ought neither to constrain nor to direct. They protect adherents of all religions, as well as those who believe in no religion at all. -- Sandra Day O'Connor, retired Supreme Court Justice