Gen 3:1-5
3:1 The serpent was clever, more clever than any wild animal GOD had made. He spoke to the Woman: "Do I understand that God told you not to eat from any tree in the garden?"
2 The Woman said to the serpent, "Not at all. We can eat from the trees in the garden. 3 It's only about the tree in the middle of the garden that God said, 'Don't eat from it; don't even touch it or you'll die.'"
4 The serpent told the Woman, "You won't die. 5 God knows that the moment you eat from that tree, you'll see what's really going on. You'll be just like God, knowing everything, ranging all the way from good to evil."

The serpent’s opinion was not what God had said so the first sin was based on an improper opinion.