[Zeth O. Grady]

"Satan" isn't an evil character in the Old Testament. He is Yahweh's "court prosecutor" who tests faithful Jews to see if they really are true worshipers of Yahweh. He is called "ha satan", which means "the adversary". He is not depicted as being an evil character at all.

It is only in the Jewish books written during the time period between the Old and New Testaments that "Satan" morphs into an evil character. This came about because Jews had incorporated many elements of the Persian religion Zoroastrianism into Judaism during the preceding centuries. In Zoroastrian theology, there is an evil anti-god called "Ahriman". This idea of an evil anti-god was slowly absorbed into Judaism and syncretized with the character of "Satan".