In Reynolds v. United States (1878), the Supreme Court said, "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.'" This was further emphasized in E verson v. Board of Education (1947), as expressed in the opinion for the majority written by associate Justice Hugo Black. He said, "The First Amendment has erected a wall between church and state. That wall must be kept high and impregnable. We could not approve the slightest breach."

In Lemon v. Kurtzman (1971), the Court established a three-prong test to determine if a governmental action is neutral toward religion. First, government institutions or legislation must have a secular purpose; second, the primary effect must be on e that neither advances nor inhibits religion; and third, there must not be an excessive government entanglement with religion.

This principle was further clarified by associate Justice Sandra Day O'Connor in Lynch v. Donnelly (1984). She said, "What is crucial is that a governmental practice not have the effect of communicating a message of government endorsement or disapp roval of religion.