Yes, parts of the Bible might also be considered representative of "natural law." What you asked for, a "codified book of natural law", is an oxymoron. If the artifact depends on a legal code, a law, a regulation, etc., then it is not natural law.
The so-called "Golden Rule" is natural law.
However, that is not really the point. The point is those who claim “natural born citizen” is defined by “natural law.”
We are asking for the page number to see if they are correct in their interpretation of the passage.