The root of the problem:

In Genesis we discover the root of the problem when Adam and Eve disobeyed God. God placed Adam in the garden and said to him:

"You may eat the fruit of any tree in the garden, except the tree that gives knowledge of what is good and what is bad.  You must not eat the fruit of that tree; if you do, you will die the same day." (Genesis 2:16 & 17)

Adam disobeyed this one command, when he listened to Eve, and ate the fruit which he was forbidden to eat.