I have sometimes used an analogy that compared a person who viewed scripture as the final word in morality to a doctor who viewed Hippocrates as the final word in medicine.
That hypothetical doctor would be considered entirely and obviously incompetent in the practice medicine in the 21st century. In fact, he would be banned from medical practice - deemed a threat to the wellbeing of would-be patients. He may claim that the writings of Hippocrates were divinely inspired and objectively true - founded on God's wisdom rather than the efforts of fallible humans. However, that would not excuse him from acting on those beliefs in ways harmful to the interests of others.
We have learned a lot in the past two thousand years. A competent physician would be expected to put that knowledge to use for the benefit of her patients - knowledge that cannot be found in and, in some cases, contradicts the writings of the ancient Greek physician Hippocrates.
Similarly, a person who holds scripture to be the final word in morality is just as incompetent to practice ethics - and for the same reasons. She, too, has to ignore everything we have learned in the past 2000 years. Her morality would be as primitive in the 21st century as the medicine of Hippocrates. She can rightly be said to be a threat to the well-being of other members of the community.
On giving this analogy some thought, I noted that we can extend it. I can imagine two different sects of Hippocratians. Both hold that the writings of Hippocrates are true and complete. However, only one group holds that the divinely inspired writings if Hippocrates are to be interpreted literally. For this group, any modern claims not found in Hippocrates represents blasphemy that must be rejected.
The other potential group of Hippocratians, in contrast, would hold that Hippocrates is not to be taken literally. They accept the claims of modern medicine, then return to the works of Hippocrates and say, "Here is where Hippocrates talks about radiation treatment for cancer. Here, we have a discussion of how mosquitos carry malaria. Here he discusses types of genetic disorders And over here we have a whole chapter on penicillin and other antibiotics - as well as information on bacteria and other microbes."
These Hippocratians might be able to practice modern medicine successfully - without being a threat to others. However, when we hear them talk about how all modern medicine can be found in Hippocrates this does give us reason to worry, just a bit. When they claim that they can find writings on antibiotics in the works of Hippocrates, what other odd ideas do they have, and how might this impact the way they treat patients? Is this going to distort their interpretation of new medical advances as they are revealed?
The moral equivalent of this species of Hippocratian are those who claim, for example, that all of our moral breakthroughs of the past 400 years of moral philosophy came from Christianity. They make claims such as saying that America was founded as a Christian nation based on Christian principles, and Christianity ended slavery.
Crediting Christianity with the end of slavery would be comparable with actually crediting Hippocrates with discovering a vaccine against Polio - by some modern physician who claims he can see all if the truths if modern medicine in the writings of Hippocrates.
In fact, if these moral truths were in scripture, why did it take 1600 years to discover them? It really would have been nice to find the words of the Declaration of Independence in scripture, and for Moses to have brought down from the mountain, not the Ten Commandments (many of which are not found in law and some of which are explicitly rejected), but the Ten Amendments. The right to freedom if speech and religion are as foreign to scripture as penicillin and radiation treatment would have been to Hippocrates.
One of the ways we can demonstrate the absurdity of the claims made by these hypothetical Hippocratians is by noting that Hippocrates is never the source of any modern medical breakthrough. These Hippocratians always discover these medical truths written in the works of Hippocraties after the fact - after they have been discovered by other methods. Furthermore, even errors, if widely believed, end up being discovered in the writings of Hippocrates.
Similarly, scripture is never a source of moral innovation.
For example, America was actually founded on the principles if the enlightenment. They cam from philosophers such as John Locke who tossed aside scripture and said that we can derive moral facts from an examination of humans in a state of nature. It was this method that revealed that humans, in nature, are equal. None have a natural right to rule or a natural duty to obey. They create governments to secure their life, liberty, and property. If any government becomes a threat to the life, liberty, and property of the people, then the people have the right to alter and abolish it - just like they have a right to alter or abolish any other tool that they create for human purposes, but which becomes a threat to their well-being.
As with an imaginary Hippocretian discovering passages about penicillin after the fact, people claim to find the moral truths on which America was founded in scripture after the fact. This only demonstrates that scripture is an object of creative interpretation. It does not show that scripture is an actual source of moral knowledge.
People who claim that they can get their moral knowledge from scripture deserve to be thought of the same way we would regard a physician who gets all of his knowledge from Hippocrates. If they truly believe that the writings of Hippocrates/scripture represent the literal and complete set if medical/moral facts as written, they are incompetent in the practice of medicine/ethics. If they make Hippocrates/scripture the object of creative interpretation, they may competently practice medicine/ethics, but this does not change the fact that Hippocrates/scripture long ago stopped being an actual source of medical knowledge.