Once the clauses reached the states the volume of litigation rose sharply, and the doctrine has been rebuilt more than once.
On establishment, the Court adopted a three part test in 1971 requiring a secular purpose, a primary effect that neither advances nor inhibits religion, and no excessive entanglement. It was applied, criticised, worked around and finally set aside in 2022 in favour of an approach grounded in historical practice and understanding, which is a substantial change of method and its consequences are still working through the lower courts.
On free exercise the swing was as large. In 1963 the Court required a compelling state interest before a general law could burden a religious practice. In 1990, in a case about the sacramental use of peyote, it held that a neutral law of general applicability does not offend the clause even if it burdens a religion. Congress responded in 1993 with the Religious Freedom Restoration Act, restoring the older test against federal action, and the states passed their own versions.
The subject matter is wide because the clauses are short. School prayer, released time, creation science, public funding of religious schools, monuments on courthouse lawns, legislative chaplains, conscientious objection, religious dress in the armed forces, the ministerial exception in employment law and the contraception mandate have all been argued under sixteen words written in 1789.