Church and State
If someone does not regard your sacred book as a source of authority, there is no point in appealing to it when trying to persuade them on a political issue. If someone said to you, "The book of Mormon says XYZ," will that persuade you that what they're saying is true? It may if you're a Mormon. But if you're not, you'll probably think the person who cited it is a crack pot. If a law is truly good, you should be able to prove it is good without appealing to a sacred text. However, if you are able to use logic and evidence to persuade someone that a law is good--and it happens to be in a sacred text--you may choose to point out that that law is in "this" text.
The video below is a good example of how people with very different beliefs can engage in political discussion. One of the reasons the discussion is so civil--despite talking about some very sensitive issues-- is their appeal to evidence and reason.