I want to know the answer to the following question:
What case was the concept relevant?
All the background I'm provided with is that "The centrality of the "separation" concept to the Religion Clauses of the Constitution was made explicit in Everson v. Board of Education, 330 U.S. 1 (1947), a case dealing with a New Jersey law that allowed government funds to pay for transportation of students to both public and Catholic schools.". Is that enough to answer the question?
Answer: yes