Genesis 3 The Fall

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.
Genesis 3:6 ESV

The serpent directly contradicts what God has said. He presents the fruit of the tree as something worth having. By eating it, he says, Adam and Eve will be like God, knowing good and evil. The irony of the serpent's remarks is that Adam and Eve, unlike the serpent, were already made in the image of God (1:26-27). They are already "like God". This means they are expected to exercise authority over all the beasts of the field--including the serpent! By obeying the sepent, however, they betray the trust that God has placed in them.

You will not surely die. It is sometimes claimed that the serpent is correct when he says this, for they do not immediately "die". Further, their eyes are in fact opened, and God acknowledges that "the man has become like one of us in knowing good and evil". Yet the serpent speaks only half-truths. What Adam and Eve will experience outside of Eden is not life as God intended. It is spiritual death.