Q.18:What was the sin whereby our first parents fell from the estate wherein they were created?
A. The sin whereby our parents fell from the estate wherein they were created was their eating the forbidden fruit.
So when the woman saw that the tree was good for food, that it was pleasant to the eyes, and a tree desirable to make one wise, she took of its fruit and ate. She also gave to her husband with her, and he ate. Genesis 3:6
Then the man said, “The woman whom You gave to be with me, she gave me of the tree, and I ate.” Genesis 3:12