Pagels charts the evolution of the Jewish and Christian concept of evil from Old Testament times to the present day (although the majority of the book deals with the New Testament era). She explains how 'Satan' didn't always refer to an evil being but was initially used to represent an obstacle. After that meaning, it evolved into a meaning which was used to unify your group against your enemies/adversaries or 'satan'. The Jews still don't, nor did they ever, believe in or create the Satan/Devil being/creature/character of Christian lore.|
It wasn't until New Testament times and later that the concept of an evil being who is actually called 'Satan' or the 'Devil' or 'Lucifer' evolved. It is interesting to see how these concepts have continued to persist throughout religious and political history with groups stigmatizing others not in their group (whether it be religious, political, racial, etc.) as being 'of the devil'. Dictatorships and other authoritarian organizations always need an external enemy to bind their followers together.