Jefferson's letter entered American jurisprudence in the 1878 Mormon polygamy case Reynolds v. U.S., in which the court cited Jefferson and Madison, seeking a legal definition for the word religion. Writing for the majority, Justice Stephen Johnson Field cited Jefferson's Letter to the Danbury Baptists to state that "Congress was deprived of all legislative power over mere opinion, but was left free to reach actions which were in violation of social duties or subversive of good order." Considering this, the court ruled that outlawing polygamy was constitutional.

When did Jefferson's letter enter American jurisprudence?