"You know Abraham probably didn't exist, right?"
You talkin' to me? You talkin' to me? No, actually, I don't know anything about the probability or improbability of Abraham's existence. Absence of evidence is not evidence of absence. I see no reason why there couldn't have been a man who lived in Mesopotamia in or around the 18th century BC and heard voices telling him to move to the area which would later become Jerusalem, where, in obedience to the same voices, or voice, he prepared to sacrifice his son, but then changed his mind and sacrificed an animal instead. None of the above strains my credulity in the slightest. It's all entirely possible. As far as how probable or improbable it all is, I don't have nearly enough data to say.
And neither do you.
Claims that Abraham was the first monotheist seem much more farfetched to me than claims that he may have existed.
Of course, when considering the historicity claims of legendary figures, there is always the question of how closely history must fit the legend before one is justified in saying that the figure existed. I don't see how there can be an absolute and objective answer to this question. If someone existed who preached everything in the Sermon on the Mount, but he existed in the 2nd century BC and was named Nathan, is he the historical Jesus, or does he prove that there was no historical Jesus? What if Nathan lived in Syria and died in Damascus? If we find evidence of a 5th century baron in Britain who was married to a Guinevere and befriended to a Lancelot, and ruled over a territory of 5 acres, have we found the historical Arthur, or proven that there was no historical Arthur? What if his wife was named Portia and their friend was named Offa? Where do you draw the line between an historical figure and an historical source of a legendary figure?
I'm only asking these questions, not offering answers to any of them.
No, actually, I will offer an answer: I'm not interested in drawing such lines, but I am intensely interested in increasing our knowledge whenever possible. Finding out what actually happened in times and places where legends began is a process of historical research, and finding out how legends grew and developed is also historical research, and the more explicitly clearly the one investigation can be distinguished from the other, the better.
Back to the story of Abraham and what may have inspired it -- what I find particularly interesting about it is the thought that the story of the near-sacrifice of Isaac may have been inspired by a culture's transition from human to animal sacrifice. Many cultures go through such a transition -- most or all cultures if James Frazer was right. Perhaps there was no historical Abraham, and the story of Abraham and Isaac, in which there was no human sacrifice whatsoever, but almost one single instance of it, may have gradually developed as a more comfortable way of remembering a time when human sacrifice was routine. Or perhaps there was a man who was about to sacrifice his son, but stopped and sacrificed an animal instead, and this one man's story was remembered and embellished because that was more comfortable than remembering that human sacrifice once was routine.