Not sure how kids are supposed to get that these days without growing up in a religious house hold.
The same way I did in my public middle school in the 90s and the same way my daughter did in her social studies class last year- by teaching comparative religion and attempting to do so without bias. And at no point was I or was she taught that the Bible was one of the important founding documents for our nation, since it wasn’t and that’s not true.
If we read any passages from the Bible or the Quran during that class, I don’t remember them. My daughter’s class did not have them. And yet we now both have enough understanding of those religions to be able to put them within a historical framework.