That is a lie, but charitably I assume you’re deceived and not a liar. Up until the early 20th century the US was overwhelmingly Christian and overwhelmingly American, which is an ethnic group of British origin. This is a scientific fact easily verified with modern genetic profiling. These are simple matters of historical fact, 1984 style history rewriting notwithstanding. I’m aware that in contemporary usage American means anyone who manages to reside in the USA, which is of course intentional because erasing the former dominant ethnic group’s identity is necessary to displace them.

The Education of Henry Adams is a wonderful autobiography by one of the old Americans who saw his people were losing their political power at that time.

Edit: yes I’m aware there were Germans, Dutch, Jews, and West Africans, among other ethnic groups present in the USA. They were however tiny minorities and not at all comparable to the post 20th century mass immigration populations. That was when the USA became a multi-ethnic and eventually multi-religious state.