Established the three part test for determining if an action of government violates First Amendment's separation of church and state:

1) the government action must have a secular purpose;
2) its primary purpose must not be to inhibit or to advance religion;
3) there must be no excessive entanglement between government and religion.

The phrase "separation of church and state" (sometimes "wall of separation between church and state"), attributed to Thomas Jefferson and others, and since quoted by the Supreme Court of the United States, expresses an understanding of the intent and function of the Establishment Clause of the First Amendment to the Constitution of the United States. The First Amendment reads "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof ....", while Article VI specifies that "no religious Test shall ever be required as a Qualification to any Office or public Trust under the United States." The modern concept of a wholly secular government is sometimes credited to the writings of English philosopher John Locke, but the phrase "separation of church and state" in this context is generally traced to an 1 January 1802 letter by Thomas Jefferson, addressed to the Danbury, Connecticut, Baptist Association, and published in a Massachusetts newspaper. Echoing the language of the founder of the first Baptist church in America, Roger Williams—who had written in 1644 of "[A] hedge or wall of separation between the garden of the church and the wilderness of the world"— Jefferson wrote, "I contemplate with sovereign reverence that act of the whole American people which declared that their legislature should 'make no law respecting an establishment of religion, or prohibiting the free exercise thereof,' thus building a wall of separation between Church & State".

Jefferson's metaphor of a wall of separation has been cited repeatedly by the U.S. Supreme Court. In its 1879 Reynolds v. United States decision, the court allowed that Jefferson's comments "may be accepted almost as an authoritative declaration of the scope and effect of the [First] Amendment." In the 1947 Everson v. Board of Education decision, Justice Hugo Black wrote, "In the words of Thomas Jefferson, the clause against establishment of religion by law was intended to erect a wall of separation between church and state." [2] However, the Court has not always interpreted the constitutional principle as meaning absolute separation of government from all things religious.