I’m not sure what relevance this has for a secular nation like the United States.
The United States government, which is what we’re discussing is absolutely a secular body. The various religions or non-religions of the citizenry are irrelevant, constitutionally.
What the first amendment doesn’t do is exclude moral beliefs rooted in religion from being the basis of public policy. If the government can regulate conduct that kills living things, then it’s immaterial whether any particular legislation is rooted in the moral framework of Christianity or Islam or Feminism. In the American system, all moral frameworks, whether rooted in religion or not, are on equal footing. That’s distinct from secularism, as practiced by France and formerly Turkey, where religious philosophy is singled out and excluded from the public sphere.
Err, no, that's a common misunderstanding.
The Establishment Clause of the First Amendment prohibits government from encouraging or promoting ("establishing") religion in any way.
It's there to forbid the creation of an official state religion (eg: the Church of England being created to replace the Catholic church back in the day).
See:
visit a cemetary and see crosses. listen to language and hear references to "christening", christian names, "this book is the x bible", expletives referencing "jesus christ", christmas, easter, the abundance of churches over any other site of worship.