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.
What does it mean to have a separation of Church and State? What is religious liberty? And can you have a free society without it? Do people have a right to be wrong?1
1. All these questions are answered in-depth in Os Guinness book, Last Call for Liberty: How America's Genius for Freedom Has Become Its Greatest Threat.
Violence in the Name of God: Mark Durie Answers Twelve Questions about Christianity and Islam
The State Should Not be the Enforcement Arm of the Church
How should unbelievers be treated?
The Parable of the Broken Window