It's a metaphor for humanity developing consciousness. Humanity lived in the garden, in ignorance and bliss. God commanded and man obeyed. Lucifer(light bringer) in Promethian fashion brought the flame of consciousness(free will) to humanity by having them eat from the tree of knowledge, and so the fall. This was the first sin, the rebellion against God. Sin is simply man being left to his own devices, forced to make his own decisions.