The separation of church and state is a legal and political principle derived from various documents of several of the Founders of the United States. The First Amendment to the United States Constitution reads "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof . . ." The modern concept is often credited to the writings of English philosopher John Locke, but the phrase "separation of church and state" is generally traced to an 1802 letter by Thomas Jefferson to the Danbury Baptists, where Jefferson spoke of the combined effect of the Establishment Clause and the Free Exercise Clause of the First Amendment. His purpose in this letter was to assuage the fears of the Danbury, Connecticut Baptists, and so he told them that this wall had been erected to protect them. The metaphor was intended, as The U.S. Supreme Court has currently interpreted it since 1947, to mean that religion and government must stay separate for the benefit of both, including the idea that the government must not impose religion on Americans nor create any law requiring it. It has since been in several opinions handed down by the United States Supreme Court,[1] though the Court has not always fully embraced the principle.[2][3][4][5][6]

The Fourteenth Amendment to the United States Constitution (Amendment XIV) is one of the post-Civil War amendments, intended to secure rights for former slaves. It includes the due process and equal protection clauses among others. The amendment introduces the concept of incorporation of all relevant federal rights against the states. While it has not been fully implemented, the doctrine of incorporation has been used to ensure, through the Due Process Clause and Privileges and Immunities Clause, the application of most of the rights enumerated in the Bill of Rights to the states. The incorporation of the First Amendment establishment clause in the landmark case of Everson v. Board of Education has impacted the subsequent interpretation of the separation of church and state in regard to the state governments.[37] Although upholding the state law in that case, which provided for public busing to private religious schools, the Supreme Court held that the First Amendment establishment clause was fully applicable to the state governments. A more recent case involving the application of this principle against the states was Board of Education of Kiryas Joel Village School District v. Grumet (1994).