Interesting example.
The significance of the (seeming) plan for Abraham to sacrifice Isaac -- which was progressed through up to the crucial moment...
...and then stopped (!)...
The significance of that sudden change -- was the deeper sea-change: no longer would human sacrifice be thought of as a way, to God's people. Instead "God will provide the lamb" as the text literally reads, and then literally shows happening. (Subsequently, the outright rule is given: no human sacrifice. e.g. Deuteronomy 12:31 (and several other instances).)
Returning back to our discussion, while I've said elsewhere that human sacrifice was common around the world, and then Israel given Law from God against it....
It faded out, over time.
That's key.
Unlike "Do not Murder", human sacrifice stopped being common...and became increasingly uncommon, and finally is now probably quite rare.....
In contrast, a rule of the form "Do not murder" (or like form) didn't fade out.... It continues, not only in the past, but continues through time.
That's one of the qualities I pointed out previously: not only common around the world, but in "any time", including recent times and the present.
I think you are doing some sort of special pleading here, in the sense that you agree that these things happened and weren't all that uncommon within cultures around the world. But rather than accepting that our moral standard have changed over time as these cultures did not stop this practice at the same time, which would easily be explained by there not being a universal law, but rather a natural development over time in our moral standards, exactly like we would expect religious beliefs to develop over time.
Like it would be amazing to find an ancient religion from a primitive hunter gathering tribe with a religion as complex as Christianity. But we don't, we find much more "primitive" ideas, such as worshipping animals, ancestors and near Earth stuff like that, which is what we would expect. And that these would develop over time to become more complex, seem to be reasonable, as these idea got questioned and maybe couldn't provide sufficient explanation for the stuff early humans experienced.
But to explain why people would stop doing it and justify objective morality you just throw in God like a savior. But this doesn't really give a reasonable answer, because human sacrifice, is only one of many bad things that humans have done and still do and can justify doing, despite of God.
Examples are slavery, genocides despite it not being human sacrifice, it is still considered murder, and have been justified by whoever did it, even after God in the Bible saw the light of day.
And a lot of these things were done by people of faith including Christians, but also people of no faith. So it seems that these things are done, regardless of the religious background a person hold.
But "Do not murder" is what started all this, human sacrifice is just part of the idea of murder, beating a slave to death or genocide are also just actions that we could put in the category of murder. And since people and societies have and can justify doing these things to other people, its difficult to explain that it is universally wrong as if it is a law.
Maybe it helps if I explain my way of approaching something like this:
So the initial question I would like to examine could be "Is morality objective, subjective or a mixture?"
First step, would obviously be to figure out what define each of these.
Next I would change the question into a condition, based on the definition - "If morality is objective, it has to be true regardless of whether we existed or not."
Finally I would define what would be valid test examples. For example, would a person which suffers from a psychological condition, which makes them incapable of making these types of judgements be a fair representation of the average humans? I don't think they will, and therefore I won't use these special cases to try to test the condition.
However if a certain action is common enough throughout societies, they will not be considered special cases. Such as child abuse which seems to happen to such a degree that it seems like a common condition of humans. This is obviously a judgement I make, which is up for debate.
Then I would start finding example to test against the condition above and will usually start with hardest questions I can think of, which I could imagine wouldn't meet the condition.
So it could be "Is it objectively wrong to murder someone?" or "Is it objectively wrong for parents to kill their child?", So will just take the last one, because that covers the first one, but is more specific.
It would be a good example to show that at least some morality is objective, if such thing had never occured. So we look for exactly such examples then.
Obviously we can look at the story of Abraham in the bible, which you also mentioned and wonder why God would want him to do it, simply to test him? and him being willing to go through with it and just before he is about to kill him, God stops him and tell him that he have proved himself. Obviously the act done by God is the working of a maniac to put a father through something like that. But that is besides the point, but what it does indicate is the thought that goes into this story and that it is considered a huge sacrifice and maybe even the ultimate test of a God. But this is easy explained away as simply being a story, so it doesn't really prove anything.
Where this gets a lot harder to explain, is when we look at actual examples from past history, this is just an example of two such cultures.
Inca culture
The Inca culture sacrificed children in a ritual called qhapaq hucha. Their frozen corpses have been discovered in the South American mountaintops. The first of these corpses, a female child who had died from a blow to the skull, was discovered in 1995 by Johan Reinhard. Other methods of sacrifice included strangulation and simply leaving the children, who had been given an intoxicating drink, to lose consciousness in the extreme cold and low-oxygen conditions of the mountaintop, and to die of hypothermia.
Maya culture
In Maya culture, people believed that supernatural beings had power over their lives and this is one reason that child sacrifice occurred. The sacrifices were essentially to satisfy the supernatural beings. This was done through k'ex, which is an exchange or substitution of something. Through k’ex infants would substitute more powerful humans. It was thought that supernatural beings would consume the souls of more powerful humans and infants were substituted in order to prevent that. Infants are believed to be good offerings because they have a close connection to the spirit world through liminality. It is also believed that parents in Maya culture would offer their children for sacrifice and depictions of this show that this was a very emotional time for the parents, but they would carry through because they thought the child would continue existing. It is also known that infant sacrifices occurred at certain times. Child sacrifice was preferred when there was a time of crisis and transitional times such as famine and drought.
There is archaeological evidence of infant sacrifice in tombs where the infant has been buried in urns or ceramic vessels. There have also been depictions of child sacrifice in art. Some art includes pottery and steles as well as references to infant sacrifice in mythology and art depictions of the mythology.
So sacrificing one's children were justified for the greater good, because it pleased the gods. If they thought it was wrong, you would assume that they would have found other things to sacrifice.
You go through enough of these and constantly run into examples of cultures and humans having done these things. Then how can we explain that it is objective morally wrong?
So we ask the same questions using subjective morality.. and it seems that these things are not difficult to explain and why some cultures would do it, while others wouldn't and why the moral standards change over time as cultures evolve.
My point being with all this, is that if we want to test an idea or belief, we have to go to the most extreme examples and see if they hold true. And apparently a God saying that it is objectively morally wrong does not explain why people would still do it.
