Although there is no doubt that the serpent deceived Eve, it is debatable whether or not the serpent actually lied. Here's why.
Then the Lord God said, "The people have become as we are, knowing both good and evil (3:22)."Earlier, the serpent told them they would become like God, knowing both good and evil (3:5).
So in verse 22, God seems to simply confirm what the serpent had said. To a certain extent, it appears as if the serpent deceived them by telling them the truth. Here's another example. First, God told Adam:
"...in the day that you eat of it [the forbidden fruit] you shall surely die."
Then later, the serpent tells them the exact opposite; "They would NOT die" (3:4). So again, it seems, at first reading, that the serpent is telling the truth and God is not. After all, they did not die in the day that they ate of the forbidden fruit.
The King James Version says:
"...in the day that thou eatest thereof thou shalt surely die.
The New International Version says:
"...for when you eat of it you will surely die.”
The English Standard Version states:
"...for in the day that you eat of it you shall surely die.”
Again, Adam did not die that very day; he lived to the ripe old age of 930 (Genesis 5:5).
So God said they would die on that very day, while the serpent said they would not die. It almost seems as if the serpent was telling the truth and God (God forbid) was telling a falsehood.
Furthermore, the serpent also told Eve that her eyes would be opened if she ate the forbidden fruit. And guess what? Just as the serpent had promised, her eyes were opened. Yet for some mysterious reason, when the eyes of Adam and Eve were opened, somehow this mysteriously caused them to suddenly become ashamed of their nakedness. So again, the serpent was true to his word. He said Eve's eyes would be opened and they indeed were opened.
Then lastly, the serpent told Adam and Eve they would somehow become like God, knowing both good and evil (3:5). And sure enough, in some sense they actually did become like God, knowing both good and evil. God even confirmed it when He said:
“The man has now become like one of us, knowing good and evil" (3:22a, NIV).
In fact, it was because Adam and Eve had somehow become like God that God banished them from the Garden of Eden. God was also concerned that they would eat the fruit of The Tree of Life and live forever.
"He must not be allowed to reach out his hand and take also from the tree of life and eat, and live forever.” So the Lord God banished him from the Garden of Eden to work the ground from which he had been taken. After he drove the man out, he placed on the east side of the Garden of Eden cherubim and a flaming sword flashing back and forth to guard the way to the tree of life (3:22b-24, NIV).