The case is Church of the Holy Trinity v. U.S., 143 U.S. 157 (1892).
The opinion can be found here:
https://supreme.justia.com/cases/federal/us/143/457/case.html
The court’s discussion of the history of the U.S. and how we are a Christian nation can be found at pages 465-471 of the opinion. This case has basically been “disappeared” by the modern atheist crowd.
Thank you sir.