Separation of church and state in the United States:

Jefferson's concept of "separation of church and state" first became a part of Establishment Clause jurisprudence in Reynolds v. U.S., 98 U.S. 145 (1878). In that case, the court examined the history of religious liberty in the US, determining that while the constitution guarantees religious freedom, "The word 'religion' is not defined in the Constitution. We must go elsewhere, therefore, to ascertain its meaning, and nowhere more appropriately, we think, than to the history of the times in the midst of which the provision was adopted." The court found that the leaders in advocating and formulating the constitutional guarantee of religious liberty were James Madison and Thomas Jefferson. Quoting the "separation" paragraph from Jefferson's letter to the Danbury Baptists, the court concluded that, "coming as this does from an acknowledged leader of the advocates of the measure, it may be accepted almost as an authoritative declaration of the scope and effect of the amendment thus secured."

Please answer a question about this article. If the question is unanswerable, say "unanswerable". What case was Jefferson's concept apropos to?
Reynolds v. U.S., 98 U.S. 145