The centrality of the "separation" concept to the Religion Clauses of the Constitution was made explicit in Everson v. Board of Education, 330 U.S. 1 (1947), a case dealing with a New Jersey law that allowed government funds to pay for transportation of students to both public and Catholic schools. This was the first case in which the court applied the Establishment Clause to the laws of a state, having interpreted the due process clause of the Fourteenth Amendment as applying the Bill of Rights to the states as well as the federal legislature. Citing Jefferson, the court concluded that "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."

When was the centrality of the "separation" concept to the Religion Clauses of the Constitution made explicit?