Part I: Questions about God, Man, and Sin
24. Q. What was the sin of our first parents?
A. Eating the forbidden fruit.
(Click through to read scriptural proofs.)
And the Lord God commanded the man, saying, “You may surely eat of every tree of the garden, 17 but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall surely die.”
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 also gave some to her husband who was with her, and he ate.
Scripture quoted from the English Standard Version.
Taken from A Catechism for Girls and Boys found here.