Cheerful Charlie
Contributor
I assume you mean by "the book" the Hebrew and Christian Bibles? The name means "the adversary" or "the accuser"; such implies that his purpose in the court of YHWH is like that of a prosecuting attourney, one whose desire is to convict humanity of our misdeeds and weaknesses and lead us away to punishment. The New Testament built on this mythos by positing a "redeemer" as well, who defends us against his accusations.
If you mean "in the history of Christianity", Satan has worn a lot of different hats over the centuries.....
I wonder at what point the perfect all-good/all-present/all-powerful God was dreamed up? It seems atheists are always looking for an omni God in Christianity and are puzzled how he's not there in the mythology.
But then how could a philosophical omni-type character be in any mythology? The point of myth, I think, is to make a story of the world. Stories have only flawed characters, no perfect ones. But such is the expectation and keeps people scratching their heads over "plot holes".
1. God is one, supreme among gods and men, and not like mortals in body
or in mind.
2. The whole [of god] sees, the whole perceives, the whole hears.
3. But without effort he sets in motion all things by mind and thought.
4. It [i.e. being] always abides in the same place, not moved at all,
nor is it fitting that it should move from one place to another.
- Xenophanes
Greek perfect being theology starts with Xenophanes.