The Tree of Knowledge was forbidden because they would die
The Tree of Knowledge of Good and Evil is often thought of in a simplistic manner. God arbitrarily forbade the eating of a specific tree as a test. It existed in the middle of the garden, but it was otherwise ordinary. Everything else special about the tree is then derived from the fact that it was forbidden. Adam and Eve would gain experiential knowledge of good and evil through the willful act of disobedience. They would die for no other reason than the sinful act in itself. This implies that because the Garden of Eden was such a perfect place, and Adam and Eve were given complete freedom to do what they wished, there was really no opportunity for Adam and Eve to rebel in any way even if they wanted. God forbade the eating of the tree to give an outlet to deliberately act on any desire inside of themselves to actively disobey God’s will for them. I call this way of thinking about the tree as the “they will die because it is forbidden” view.
The other view is that the tree was already inherently special apart from being forbidden. In this view, the Tree, as created, had the inherent quality of being “of the Knowledge of Good and Evil.” Eating the fruit of a Tree with this particular innate quality would bring death. I call this the “it was forbidden because they would die” view.
This still leaves the question open as to why God created such a tree in the garden, but this is not a question we need to answer. God created this tree for His own purposes and forbade Adam and Eve from eating of it for their own good. Eating the fruit of this tree would still be an egregious act of rebellion, but it was not forbidden as a pure test. What reasons are there to conclude that the Tree was special apart from, and independent of, its status of being forbidden?
It is called “the Tree of the Knowledge of Good and Evil” in Genesis 2:9 before eating its fruit is forbidden in Genesis 2:17. This implies that it had this quality before being forbidden, but it could have retroactively been called that name.
There is another special tree in the garden, the Tree of Life. That tree was not forbidden, so it could be understood that that tree has the quality of being “of life” in an innate sense. So it would stand to follow that the other tree also had the quality of being “of the knowledge of good and evil” in an innate sense.
Scripture does not explicitly say what it means for a tree to have the quality of being “of the knowledge of good and evil” but the name itself strongly implies that eating the fruit would instill knowledge of good and evil on the person eating. The state of affairs before eating the fruit is interesting to think about. Was living in the garden hazardous? Was there a constant risk of Adam randomly punching Eve in the nose because he had no idea he was not supposed to do that? No. Adam may not have had a conception that it was “wrong” to punch Eve in the nose, but he knew it would be painful for her and possibly injure her. Adam loved Eve as he loved himself. Jesus said, “Love worketh no ill to his neighbour: therefore love is the fulfilling of the law” (Romans 13:10 KJV). Adam and Eve naturally acted in the best interests of each other out of love without knowing what “good” and “evil” were.
To further explore this, imagine Adam and Eve were standing oat an overhang admiring the sunset. Satan whispers in Adam’s ear, “an evil man would push a woman off the ledge if it suited him. He would not give a thought to if he ought to do that or not, he just would go ahead and push.”
Adam would look down and see a ten foot drop with bushes at the bottom. Adam would think, “I do not know about this ‘evil’ you are speaking of, but such an action would be profoundly stupid. Eve would be seriously hurt!”
Adam would be looking at the situation the exact same way as if Satan said, “An evil man would jump right off the ledge if it suited him. He would not give a thought to if he ought to do that or not, he just would go ahead and jump.”
Adam would not begin to imagine how a man in his right mind would decide it suited him to jump naked ten feet into a bunch of bushes. I would make no sense. It would make just as little sense to imagine pushing someone else off. Adam would be judging the action on a scale of wisdom and foolishness out of love for his neighbor Eve rather than on a scale of good and evil.
How does acquiring knowledge of “good” and “evil” spoil this idyllic setting? In many ways. This time, when Satan whispered in Adam’s ear, Adam would be able to see things from the perspective of he evil man. He could imagine what it would be like to be walled off in selfishness to the point where pushing another person off the ledge would be conceivable. This could easily lead to pride. “An evil man may very well push someone off, but I never would, because I am so good.”
Adam would no longer be making decisions purely out of love based on what would be wise or foolish, he would be making decisions in a detached manner based on a standard of what would be “good” or “evil” to do. Adam and Eve’s outlook completely changed for all of mankind. Now that we know what good and evil are, we cannot pretend like we do not. We now need the ten commandments where they would have been unnecessary in the Garden of Eden. After eating the fruit, Adam and Eve knew they were naked and covered themselves. They could imagine themselves behind the eyes of a selfish person looking at their exposed bodies and they were ashamed. God did not tell them to take their clothes back off because they were being silly. God made suitable garments for them. The paradigm shift was tragically real.
Ever since then our relationship to the law has been complicated (see Romans, Galatians, James, etc.). The law is good, but is part of the tragedy of our existence that we need it instead of naturally doing what is wise out of perfectly loving our neighbors as ourselves.