What was the sin whereby our first parents fell from the estate wherein they were created?
The sin whereby our first parents fell from the estate wherein they were created, was eating the forbidden fruit. (Shorter Catechism, A. 15)
In the beginning, God create man male and female, and he blessed them, and he said to them to be fruitful and multiply, to fill the earth and rule over all that God created on the earth. (Gn 1:27-28)
God gave to them for food, any tree, except the tree of the knowledge of good and evil. God warned the man, that breaking the commandment would result in death. (Gn 2:16-17)
Having listened to the lies of the serpent, the fruit of the tree delighted the eyes of the woman, so she ate, and she gave also to her husband with her and he ate. (Gn 3:4-6)
Thus, original sin, and all sin, is going away from the original estate in which God created mankind.