Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof ...—First Amendment
In the words of Jefferson, the clause against establishment of religion by law was intended to erect "a wall of separation between church and State."—Justice Hugo Black
Can I smoke peyote as part of my religion (even if it violates Federal drug laws)? What about practicing bigamy? Do I have to pledge allegiance to the flag if my religion forbids it? And what about prayer -- can I do it in a school? What about at a town council meeting? In deciding these kinds of cases, the Supreme Court has, over time, developed principals to regulate conflicts between religion and law.
Religious freedom is guaranteed by the U.S. Constitution in the First Amendment as quoted above. It has two components, corresponding to the two clauses in the amendment's text: the separation of church and state, which is proclaimed in the Establishment Clause ("Congress shall make no law respecting an establishment of religion") and the freedom of each individual to exercise their religion, expressed in the Free Exercise clause ("or prohibiting the free exercise thereof"). This dual guarantee protects the individual from a government institutionalization of religion, and at the same time protects individuals from a government hostile to their religion. However, the fifteen word guarantee in the First Amendment wouldn't reach it's full potential until the 20th century.
Free Exercise
The freedom of religion is guaranteed by the Constitution in the First Amendment. The text of the constitutional prohibition against religious establishment is called the Establishment Clause; the text of the guarantee of the "free exercise" of religion is the Free Exercise Clause. Initially, these First Amendment freedoms meant that the national government could not abridge the religious rights of individuals. In the early 20th century, these freedoms were "incorporated against the states" through the Fourteenth Amendment (see Incorporation).