The First Amendment to the U.S. Constitution says only that "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof..." In 1802, Thomas Jefferson wrote that its declaration, known as the "Establishment Clause," has built "a wall of separation between Church & State."
In 1947, Jefferson's interpretation was cited in a Supreme Court decision allowing public funds to pay for buses taking children to Catholic schools. Justice Hugo Black added that the “wall must remain high and impregnable.” "This discussion had a decidedly separationist tone and has been cited by liberals as authoritative ever since," observes Artemus Ward, a Northern Illinois University professor.