I have had many circular discussions with religious authoritarians over the years. I can pretty much sum them all up as follows:
RA: God's Law must be the law of the land!
ME: How do you even know what God's law is?
RA: It's in the Bible/Koran/Book of Mormon/Other
ME: How do you know that your book is actually a message from God?
RA: It says so right in it!
ME: If I handed you a book that I wrote that contained a sentence declaring it the Word of God, would you believe it?
RA: Of course not.
ME: Why should I or anyone else accept your book as the Word of God just because you say it is?
RA: Because my book really is the Word of God.
ME: What if I told you that I believed that this other book over here is the actual Word of God?
RA: Then you'd be wrong.
ME: Says you.
RA: Says God!
ME: Sorry, you are not God. Your book is a bunch of hooey, and mine is the Word of God, so I reckon you'll have to follow God's Law as laid out in my book and not in yours. God told me that my book was true and that yours was a trick of Satan. Or at least that's what I'm going to write in my book when I get around to it.
RA: You are so going to hell.
In sum, religious proposition A is true because the authoritarian believes that it is true; therefore, everyone else must be compelled to accept its truth, by force if necessary.