By what sin did our first parents from their original condition?
Our first parents' sin was eating the forgotten fruit.
To say that our first parents sin was merely eating the forbidden fruit, is an understatement of galactic proportion. While their sin was eating the forbidden fruit, the implications of their actions was far greater.
In the creation story, it would be hard to believe that God had acted in any other manner toward man, other than pure goodness. He created perfect world, and placed man in the center of a beautiful garden. He entrusted man as the steward of all of creation. He gave man the authority to name each of the creatures in His amazing creation. When man was found to be lacking a suitable partner, God gave him a helpmate or the perfect partner to him. To see God's character as anything but gracious toward man would be ridiculous. The goodness of God toward man is what makes the sin of eating the forbidden fruit so outrageous.
In that first act of treason, our first parents rejected God's own self-revelation. God had displayed Himself to be all good, and yet our first parents rejected that, and believed that He must be holding out on man. They decided that the goodness of God toward them was not perfect. There was more that God had reserved for Himself. God, in the eyes of our first parents, was good, but he was not perfectly good.
Secondly, our first parents denied God's supremacy. The lie that they believed concerning the fruit would mean that God was in His position because he held back knowledge from man, and not because he was the One Supreme ruler of the universe. They believed that, were they to eat the fruit, that they would be equal to God. They denied the uniqueness of God. He merely had more information and that was the only thing that separated man from God.
Our first parents, in their disobedience, created a separate rule for themselves. They place themselves in a position of being able to chose between right and wrong. As the Creator of everything, God reserved the single authority to decide right and wrong. Man, in his disobedience, established a rival kingdom, in which man would serve as the ultimate authority of right and wrong. The divine character of God could not allow such a kingdom to exist. God needed to exercise his divine prerogative and authority