Separation of church and state comes from the First Amendment to the U.S. Constitution. The amendment itself never uses that phrase, but its two religion clauses do the work the phrase describes: one bars the government from establishing or sponsoring religion, and the other protects your right to practice your faith freely. Ratified in 1791 as part of the Bill of Rights, the First Amendment has been the constitutional foundation for church-state law ever since.1Congress.gov. U.S. Constitution – First Amendment
The phrase “separation of church and state” itself traces to a letter Thomas Jefferson wrote in 1802, and the Supreme Court did not adopt it as a legal metaphor until 1947. The concept and the wording have different histories, and both matter for understanding what the First Amendment actually does.
The Two Clauses That Do the Work
The First Amendment opens with two distinct protections concerning religion. The Establishment Clause prevents the government from creating an official religion, favoring one faith over others, or backing religion generally over nonbelief. The Free Exercise Clause protects your right to hold religious beliefs and to act on them.1Congress.gov. U.S. Constitution – First Amendment
Together, these two clauses produce what people call separation: the government stays out of religion, and religion stays free from government control. They can pull against each other in practice. A strict reading of the Establishment Clause might push a state to exclude religious groups from a public program, while a strict reading of the Free Exercise Clause might require including them. Much of modern church-state law involves courts drawing lines between the two.
What the Establishment Clause Prohibits
In practice, the Establishment Clause means government bodies cannot mandate prayer, fund religious worship, or design programs to channel public money to religious organizations for specifically religious purposes. It applies to Congress, state legislatures, city councils, school boards, and every other level of government.
What the Free Exercise Clause Protects
The right to believe is treated as absolute. No government action can reach into your mind and penalize what you think about God or religion.2Congress.gov. Amdt1.4.1 Overview of Free Exercise Clause Religious conduct is a different matter. A neutral law that applies to everyone can generally be enforced even against someone whose religion conflicts with it, under the rule from Employment Division v. Smith (1990).3Justia U.S. Supreme Court. Employment Division v. Smith, 494 U.S. 872 (1990) When a law is not neutral or not generally applicable, courts apply strict scrutiny and require the government to show a compelling reason for the restriction.
Where the Phrase “Separation of Church and State” Comes From
The actual words trace to a letter Thomas Jefferson wrote on January 1, 1802, to the Danbury Baptist Association in Connecticut. Jefferson described the First Amendment’s religion clauses as “building a wall of separation between Church & State.”4Founders Online. Thomas Jefferson to the Danbury Baptist Association, January 1, 1802 He was responding to Baptists who feared that Connecticut’s established Congregationalist church threatened their religious freedom.
Jefferson’s metaphor sat largely dormant in legal circles for nearly a century and a half. The Supreme Court adopted it in 1947 in Everson v. Board of Education, which used the “wall of separation” language to describe the constitutional boundary between government and religion.5Justia U.S. Supreme Court. Everson v. Board of Education, 330 U.S. 1 (1947)
The metaphor has drawn criticism from several Supreme Court justices. Justice Rehnquist argued in Wallace v. Jaffree (1985) that there was “simply no historical foundation” for the wall Everson described. The phrase remains embedded in everyday language, but its role as a legal test has faded.
How the First Amendment Reaches State and Local Governments
The First Amendment originally restricted only the federal government. Congress could not establish a religion, but states technically could, and several did in the early years of the republic. That changed through a legal process called incorporation, rooted in the Fourteenth Amendment’s guarantee that no state can deprive a person of liberty without due process of law.6Congress.gov. Fourteenth Amendment Section 1 Due Process Generally
The Supreme Court applied the Free Exercise Clause to the states in Cantwell v. Connecticut (1940), declaring that “the Fourteenth Amendment has rendered the legislatures of the states as incompetent as Congress” to pass laws restricting religious freedom.7Justia U.S. Supreme Court. Cantwell v. Connecticut, 310 U.S. 296 (1940) Seven years later, Everson v. Board of Education extended the Establishment Clause to the states as well. Since then, both religion clauses have applied at every level of government. A city council that mandates prayer at its meetings faces the same constitutional scrutiny as an act of Congress.
How Courts Apply the First Amendment Today
For roughly half a century, courts analyzed Establishment Clause cases using a three-part framework from Lemon v. Kurtzman (1971). A government action had to serve a secular purpose, could not primarily advance or hold back religion, and could not excessively entangle the government with religious institutions.8Justia U.S. Supreme Court. Lemon v. Kurtzman, 403 U.S. 602 (1971) The Supreme Court formally abandoned that test in 2022.
In Kennedy v. Bremerton School District, the Court ruled that Lemon and its offshoots are “no longer good law” and held that the Establishment Clause must be interpreted by reference to “historical practices and understandings.”9Justia U.S. Supreme Court. Kennedy v. Bremerton School District, 597 U.S. (2022) Under this approach, the question is whether a government action involving religion has a historical precedent in American tradition. Practices accepted around the time the First Amendment was adopted weigh heavily in favor of constitutionality. The shift matters because the old Lemon test often led courts to strike down government actions that touched religion, while the history-and-tradition approach is more permissive toward longstanding practices like legislative prayer, religious monuments, and public acknowledgments of faith.
The Court signaled this shift a few years earlier in American Legion v. American Humanist Association (2019), which upheld a large cross-shaped war memorial on public land on the reasoning that the passage of time had given it historical and cultural significance beyond its religious origins.10Justia U.S. Supreme Court. American Legion v. American Humanist Association, 588 U.S. (2019)
A parallel line of cases has pushed against strict-separation readings of the Establishment Clause in the funding context. Espinoza v. Montana Department of Revenue (2020) held that a state creating a scholarship program for private schools cannot exclude religious schools just because they are religious.11Justia U.S. Supreme Court. Espinoza v. Montana Department of Revenue, 591 U.S. (2020) Carson v. Makin (2022) extended that rule to Maine’s tuition assistance for students in areas without public high schools.12Justia U.S. Supreme Court. Carson v. Makin, 596 U.S. (2022) A state does not have to fund private education, but if it chooses to, it cannot cut religious schools out of the program solely because of their religious character.
On the Free Exercise side, Fulton v. City of Philadelphia (2021) sharpened the rule from Smith. A policy is not “generally applicable” if it includes a mechanism for granting individualized exceptions. Once the government can make exceptions for some, it cannot refuse to accommodate religious believers without a compelling reason.13Justia U.S. Supreme Court. Fulton v. City of Philadelphia, 593 U.S. (2021)
The First Amendment is the constitutional source of separation of church and state, but it is not the only source of religious freedom law in the United States. Federal statutes like the Religious Freedom Restoration Act and the Religious Land Use and Institutionalized Persons Act add layers of protection, and Title VII of the Civil Rights Act extends religious protections into private workplaces. Those statutes sit on top of the First Amendment; they do not replace it, and they can be repealed. The First Amendment cannot.