Remember the movie, the Wizard of Oz (1939)? Remember Kansas, which was depicted as the heart of America in that movie?
Remember Auntie Em? Remember that SHE was a Christian? Why did they mention that? They mentioned it because it was something that pretty much all American audiences could relate to.
Even as a kid in the 1950s, I could relate to it, and my family was Jewish!
Auntie Em states the fact of her Christian faith.".
How well would it have connected with American movie audiences if she had said: "I'm an atheist." Or "I'm a Muslim?" OR even "I'm a Jew."
And don't even get me started on Elvis Crying in the Chapel.
It is impossible to read almost anything about America before 1990, or observe pretty much any expression of mainstream culture, and not draw the conclusion that America was fundamentally a Christian nation.
Yes. If you emphasize the word "was", that is certainly true.