Then why appeal to them at all?
This has something that has always ruffled the feathers a bit. I have never understood why/how logically a person can hold that the Scriptures of the Old and New Testaments were put together by men with an agenda and sometimes men with no real purpose (other than of course to make conservatives look foolish 20 centuries in the future) and yet still when asked why they believe x,y, or z they appeal to the Scriptures as if they have authority.
Person A is in a discussion with Person B.
Person A is a person arguing for a religious solution from a liberal perspective.
Person B is a person who holds a contrary position.
Let us say they are discussing feeding the poor.
Person A says the Civil Magistrate should redistribute wealth in order to pay for feeding homeless person A and needy person B.
Person A cites Isaiah 1:17 in defense of their position.
Person A believes “Isaiah” was written by as many as 4 different persons (or more) and certainly is post-exilic and was definitely not written by some “prophet” named “Isaiah”. Person A may even believe “Isaiah” was a partisan-piece written for Israelite zealots.
So the question is how can Person A cite Isaiah as in any sense authoritative in making their argument?