This is the
original sin, and concerns
the nature of evil, and its purpose. It is sometimes called just
the fall.
This is the event that sets the whole Christian story going. It is the foundation of free will which, in the belief of many, is the gift of god to man and woman.
It allows us to choose good or evil, and not follow some devinely prescribed path. Rather like the events in Clockwork Orange, where Alex is conditioned not to do evil, and coincidentally, not to like Ludwig Van Beethoven--but not by choice.
As the Bascilian father, and my professor, Arthur Gibson used to say, this is the second of the four great Christian themes that resonate throughout history:
Creation, Sin, Redemption, Transfiguration
See WickerNipple's Julian of Norwich. For a curious counterpoint to this, see Frankfurt example.