"In Christian doctrine, the Fall of Man, or simply the Fall, refers to the transition of the first humans from a state of innocent obedience to God to a state of guilty disobedience to God. In Genesis chapter 2, Adam and Eve live at first with God in a paradise, but the serpent tempts them into eating the fruit from the Tree of Knowledge of Good and Evil, which God forbade. After doing so they become ashamed of their nakedness and God consequently expelled them from paradise. The Fall is not mentioned by name in the Bible, but the story of disobedience and expulsion is recounted in both Testaments" (Wikipedia, "Fall of Man").
In his address "Is Theology Poetry?" Lewis states, "The earliest stratum of the Old Testament contains many truths in a form which I take to be legendary, or even mythical" (The Weight of Glory). So I am not sure what his views were on the literal truth of the story of Adam and Eve. If one does away with Adam and Eve as literal people, however, the Fall of Man becomes problematic.