Judges, lawyers, and scholars all cite the 1947 case, Everson v. Board of Education,
as the cornerstone of Establishment Clause doctrine. They are right to do so. The Everson Court took two path-breaking steps: incorporation and strict separation. The justices there held, for the first time, that the Fourteenth Amendment made the Establishment Clause applicable to state action. Then they offered a novel account of what non-establishment meant. In sweeping language (which you can find at 330 U.S. 15 - 16) they held that it meant more than equality among religions. By requiring that all government authority in the United States be neutral as well between religion and non-religion, the Everson
Court called for a secular public square.