Yale University is hosting a free course: Introduction to the Old Testament. An excerpt from a lecture 3:

So, one of the things I’ve tried to claim in describing Genesis 1 is that in this story evil is represented not as a physical reality. It’s not built into the structure of the world. When God rests he’s looking at the whole thing, [and] it’s very good, it’s set up very well. And yet we know that evil is a condition of human existence. It’s a reality of life, so how do we account for it? And the Garden of Eden story, I think, seeks to answer that question. It actually does a whole bunch of things, but one thing it does, I think, is try to answer that question, and to assert that evil stems from human behavior. God created a good world, but humans in the exercise of their moral autonomy, they have the power to corrupt the good. So, the Garden of Eden story communicates what Kaufman would identify as a basic idea of the monotheistic worldview: that evil isn’t a metaphysical reality, it’s a moral reality. What that means ultimately is that evil lacks inevitability, depending on your theory of human nature, I suppose, and it also means that evil lies within the realm of human responsibility and control.

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It is true — and maybe this will go a little bit of the distance towards answering it — it’s by eating of the fruit in defiance of God, human beings learn that they were able to do that, that they are free moral agents. They find that out. They’re able to choose their actions in conformity with God’s will or in defiance of God’s will. So paradoxically, they learn that they have moral autonomy. Remember, they were made in the image of God and they learn that they have moral autonomy by making the defiant choice, the choice for disobedience. The argument could be made that until they once disobeyed, how would they ever know that? And then you might raise all sorts of questions about, well, was this part of God’s plan that they ought to know this and should know this, so that their choice for good actually becomes meaningful. Is it meaningful to choose to do the good when you have no choice to do otherwise or aren’t aware that you have a choice to do otherwise? So, there’s a wonderful thirteenth-century commentator that says that God needed creatures who could choose to obey him, and therefore it was important for Adam and Eve to do what they did and to learn that they had the choice not to obey God so that their choice for God would become endowed with meaning. That’s one line of interpretation that’s gone through many theological systems for hundreds of years.

So the very action that brought them a godlike awareness of their moral autonomy was an action that was taken in opposition to God. So we see then that having knowledge of good and evil is no guarantee that one will choose or incline towards the good. That’s what the serpent omitted in his speech. He said if you eat of that fruit, of the tree of the knowledge of good and evil, you’ll become like God. It’s true in one sense but it’s false in another. He sort of omitted to point out… he implies that it’s the power of moral choice alone that is godlike. But the biblical writer will claim in many places that true godliness isn’t simply power, the power to do what one wishes. True godliness means imitation of God, the exercise of one’s power in a manner that is godlike, good, life-affirming and so on. So, it’s the biblical writer’s contention that the god of Israel is not only all-powerful but is essentially and necessarily good. Those two elements cannot become disjoined, they must always be conjoined in the biblical writer’s view. And finally, humans will learn that the concomitant of their freedom is responsibility. Their first act of defiance is punished harshly. So they learn in this story that the moral choices and actions of humans have consequences that have to be borne by the perpetrator.