The same principles that guide every-other religion.  I happen to think it was clear that the Founders wanted our nation to be based on ethical principles, but struggled with how to define that, just as religions do.  So religions base their principles on the morals of men, as espoused in some holy book or other (which they interpret to their liking), instead of basing their principles on the laws of the Universe, which are immutable and not based on any book.

I attended a private, Catholic school in junior high, but attendance was not limited to Catholics.  Those of us who were Catholic had to take catechism, while those who weren't Catholic took a class in ethics.  I happen to think the non-Catholics got the better explanation of right vs. wrong, and how to treat people.