It seems to me, iambiguous, that your assumptions are:
(a) an omniscient entity can devise a non-independent but objective system of ethics,
(b) humans will never have the capacity to do this
(c) without an omniscient entity, there can be no objective system of ethics
As to (a), well, we've spoken about it a lot already. You say, for example:
If God is omniscient...if He knows everything...doesn't it follow necessarily that his moral prescriptions are derived from the most rational manner in which to think about them?
Well, if god thinks about it in a rational manner, then there must be some logic which he or she follows. If the system is consistent, then there is some consistent logic that underlies the system of ethics. If god is totally rational and doesn't put any of his own subjective opinion in, then this is an objective system that does not require god in order to exist - it is logically consistent in its own right. If god does put his own opinion in, then the system is not objective, so it does not automatically follow that an omniscient god leads to an objective system of logic, unless there is a logic that god can follow and use.
As I understand it...from the perspective of the God of Abraham...He granted Adam and Eve both a paradise on earth free of punishment and free will. They used their free will to disobey God, and brought punishment upon themselves and all the rest of us. Original Sin, in other words. This whole thing is preposterous to me but not to others. Yes, many do worship and adore God because they fear His retribution. But at least they have a Scripture that allows them to differentiate right from wrong objectively. Depending, of course, on how this is interpreted by the different denominations.
Well, this just shows that you (i) don't believe there is evidence for an objective system of ethics invented by a god, (ii) that you agree it is based upon a system of punishment and reward, not necessarily related to a system of objective ethics, and (iii) that you recognise others only think it is objective from their perspective.
As to (b): if a logical system of ethics exists, can humans ever understand this?
Anything is possible I suppose but when abortion as a medical procedure was in its infancy men and women were arguing about the morality of it pretty much as we still are today. That, to me, speaks volumes. Why no progress at all?
Why no progress at all? Well, even on established fact (such as evolution or the theory of special relativity) people in the past and today disagree. So disagreement is not evidence of lack of progress. And, as I stated earlier, at some point in time every discovery is new and so takes up less than 1% of history. Should ethics follow the same time scale as medical history or the physical sciences? It was only last century that mathematicians really stopped taking maths for granted and delved into set theory and others. So you can judge the progress of this, but it is a subjective judgement (unless you come up with a full-blown "theory of discovery").
Are humans capable of understanding an objective system of ethics if it exists? Well, we don't know, so we'll have to stick to our subjective judgments about that as well. But what we can say is, If humans can never understand it, then we can simply put it into the category of "doesn't exist" because it is practically the same.
However, I think you give humanity a raw deal when you assume both (i) people can never transcend or understand themselves in relation to history (i.e. be totally defined by history) and (ii) that culture itself will never create a situation where people might discover objective ethics. They are pretty big assumptions.
What about (c)?
But if there is no God there is no moral logic. We're on our own. And the closest we can get perhaps to "objectivity" is to embrace the points folks like Harris propagate about Bad Lives and Good Lives, about "the well-being of conscious creatures."
This quote is just subjective opinion. There needs to be some sort of chain of reasoning or evidence to show that "if there is no God there is no moral logic". As a claim on its own it means very little.
But my point is that regarding questions like this there is no "distinct chain of reasoning" one can use to answer them one way or another.
What this really means is, "I cannot think of a distinct chain of reasoning one way or the other." I have put forward the beginning of my chain of reasoning: in regards to free will, either we have some free will or we don't, and where free will is applicable, there ethics is applicable. Ethics is contingent on free will. I will actually have more to say about this in another post, but the point I want to make here is that being unable to think up a chain of reasoning does not prove that a chain of reasoning does or does not exist. If you are going to make the claim "There can be no objective ethics without god?" I feel it is fair to provide a chain of reasoning both that there is objective ethics with god and a chain of reasoning that shows how there is no objective ethics without god. I think "because it is complex and I cannot imagine it" is not enough evidence to back up a claim.
But until it can be shown how an ethics sans God can be more than merely shared [and conflicting] narratives, I'll continue to endorse situational ethics regarding most of the conflicts that rend us.
I heartily agree. We should do the best we can until we know better.
I hope this clears up some of my thinking, and where I disagree that some things you find obvious and intuitive are actually fact.