I recently came across "Was America Founded As a Christian Nation?: A Historical Introduction" by Fea, and I was wondering what the state of the professional historical understanding in this regard was today. A search on this subreddit suggests that the question has been asked before, a long time ago, and the answers weren't really held to the current standards of moderation.
Part of my musing is whether the USA, in the sense of the people that have talked about this question, is conventionally understood to be grounded in the Pilgrim settlements or in the writing & ratifying of the Constitution / Articles of Confederation. Clearly the Pilgrims have a very different set of beliefs from the writers of the Constitution.
Fea's book clearly is a survey on the second time and group, but I'm not persuaded that Americans always mark the Constitution as the birthplace of the nation. So, I think, part of the answer to the question "Was America Founded as a Christian Nation" involves the answer to the question over time "When and who founded the USA".
I'm highly aware of the contentious nature of this question: please don't start a flamewar - I am interested in the specialist historian's understanding (subreddit rule:" No political agendas or moralising").
And, of course, good books recommended to survey on the matter are always welcome.
Thank you for your kind consideration.
Another aspect to consider is why you are fixated on the Pilgrims specifically when discussing the original colonists. The Separatists were neither the first nor ever a majority of colonists. Even English colonists. Virginia is nearly two decades older, and the rest of America's original states were formed like it, more in the vain of profit than prophecy.
But your confusion points to a very real phenomenon. The Pilgrims have ascended to key status in America's founding myth. Why is that? I could make some guesses, but I'd prefer to leave that question to our historiographers. How did the Pilgrims come to form such an important part of America's mythic nationhood? How early in the 19th century did that take off? To what extent does it continue?