God created man and woman in his image (Gen 1:27), but then the serpent tempted the woman by saying she would be like God if she ate from the tree God warned not to eat from. So even though she was already like God (created in his image), she was tricked into believing she would gain something desirable that she didn’t already have. But instead the opposite happened – by following the guidance of the serpent (to eat from the tree) instead of the guidance of God (to not eat from the tree), the man and woman at that instant were no longer living by faith in God and instead were following the devil and his deception (and now living in fear). God warned the man and woman of the consequences (death) of eating from that tree and warned them of further consequences when he confronted them just after the fall (patriarchy, hardship, etc).
Only by refusing the devil (through Christ), repenting of our fallen ways, and again living by faith in God can we eat from the tree of life, which is true wisdom and understanding (Proverbs 3:18-19), and live forever. The serpent made the fruit of the tree God warned not to eat from look “desirable as a way to gain wisdom” (Gen 3:6), but actually wisdom is found in following God, which was what man and woman were already doing before they ate from the tree God warned not to eat from. Eating from it caused them to lose wisdom they already had rather than gain wisdom they thought they lacked. This is the lie and the deception of the serpent from the beginning that Jesus spoke about in John 8:44.