Zoroastrianism: "Of all the other nine extra-Biblical living religions, Zoroastrianism is the only one from which a definite religious belief has been borrowed and included in the Bible. Consistently throughout the Old Testament down to and including the Isaiah of the Exile, the ultimate source of everything, including evil, is represented as the God Jehovah. But a distinct change took place after the Exile. A comparison of two parallel accounts of a certain experience of King David will show that a post-exilic document (1 Chronicles 21:1) substitutes 'Satan' for 'Jehovah' in the pre-exilic account (2 Samuel 24:1). Thus Satan is not an original feature of the Bible, but was introduced from Zoroastrianism."
Had some interesting conversation about this in my World Religions class today. Most people who know a bit about Christianity seem to disregard the history of the religion, thinking that the way it is now (or the way it was in 4th Century or at the time of the Reformation or at the time of America becoming a nation) is the way it has always been.
Attributing theological notions to places outside of the Judeo-Christian tradition does not cause me to doubt God or not want to follow Jesus. In fact, it allows me the thought that (here it comes... dun, dun, dun...) God was at work in other traditions around the world. Chew on that one.