Light and darkness are two of the oldest and most universally recognized symbols in the world. Nearly every culture and religion in history have some sense of “light versus darkness” as a root metaphor, a symbol of good versus evil, and the Judeo-Christian tradition is no exception. Consequently, there is debate regarding the proper background against which to understand the use of “light and darkness” imagery in the Gospels, and in John’s Gospel in particular.
(Dictionary of Jesus and the Gospels)