Q: What was the sin whereby our first parents fell from the estate wherein they were created?
A: The sin whereby our first parents fell from the estate wherein they were created, was their eating the forbidden fruit.
Genesis 3:6: So when the woman saw that the tree was good for food, and that it was a delight to the eyes, and that the tree was to be desired to make one wise, she took of its fruit and ate, and she gave some to her husband who was with her, and he ate.
Why was it bad for Adam and Eve to eat the fruit? First let's say why it was not bad to eat the fruit. It was not bad because the fruit itself was somehow bad. We know that all of God's creation was good. God himself, upon looking at his work, said it was good. The fruit had no evil in and of itself. It wasn't vile, it wasn't poisonous, in fact there probably wasn't anything distinct about this fruit at all. One commentator said you could have walked by this tree and its fruit a hundred times without ever noticing it.
This command wasn't given as a means of protecting Adam and Eve from evil, or from keeping them from becoming wise, rather, the command was given, as John Calvin puts it, as a "trial of man's obedience". The big deal here, what made Adam and Eve's decision to eat the fruit so bad, was because in doing so they were intentionally and outrightly rebelling against the law of God. Augustine said that the root of this sin was pride, the worship of ourselves. Why does he say this? Because ultimately, by breaking the law of God, Adam and Eve were saying that their wants, their desires, their will took primacy over the will of God. It is as if they said, "We know what God's will is, we know obedience to his will is important, but we know what our will wants, and it is more important to serve our will." They elevated themselves to a position above God himself, and each and every time we sin, we rebel, we break the law of God, we are doing the exact same thing. We are worshiping ourselves, desiring to glorify ourselves and fulfill our own wants and desires rather than glorifying God and desiring to serve his will.