Did you know that we have the Zoroastrians to thank for the good/evil polarity that most Christians now ascribe to God and Satan (i.e. God is only and all good, Satan is only and all evil)? Most of the Pentateuch was written between 900 and 540 BCE; Zoroas ter lived around 600 BCE and was influential in shaping Christian theology in later centuries.
So here's the thing about the Pentateuch: as this website puts it, "There are no passages within the older parts of the Hebrew Scriptures where Satan is portrayed as an evil devil - the arch enemy of God and of humanity. At most, he is described as a henchman who carries out God's evil instructions. There is no dualism here between two powerful supernatural entities: an all-good God and an all-evil Satan. God is portrayed as performing, directly and indirectly, both kind and evil deeds." (See especially Isaiah 45:6-7 and Lamentations 3:37-38.)
Then Zoroaster came along, with remarkable similarities to Jesus' story. From the same website: "Like Jesus, he was recorded as having been tempted by Satan; he performed many miracles and healings and was considered a supernatural being by his followers. He introduced a major spiritual reform and created what is generally regarded as the first established monotheistic religion in the world."
Zoroaster promulgated the idea that God was all good and had a twin brother who was the "God of Evil." It seems that the polarity of God and Satan developed within the Hebrew/Christian religion only after Zoroaster's time, and has today become a cornerstone of the Christian story.
I find this historical arc of what is a major tenet of modern Christianity totally fascinating. And it really discredits, for me, the idea that the Bible contains "eternal truths" and shows us the "unchanging nature" of God. Think of how modern Christians bend over backwards to justify and otherwise explain how God ordering genocide in the OT is fundamentally a "good" act. Those who wrote the Hebrew scriptures apparently didn't believe the nature of God to be fundamentally or wholly good, and they were okay with it.
So why are modern Christians trying to "rewrite" who God is? How is their version of who God is any closer to the truth when you consider that it seems to be borrowed from another religion altogether? If it's not a "rewriting" of God's character and is instead a revelation or development in our understanding of God, the obvious question to me is: why would God 'reveal' himself to one group of people as author of both good and evil, but 'reveal' himself to another group as only good?
I used to be a huge believer in a literal Satan and for a time was into book series like Left Behind. C.S. Lewis's Screwtape Letters scared the bejeezus out of me. But when I experienced some mental illness (depression), it just didn't ring true to believe that it was because of sin or because Satan was trying to distract me from God. My depression had its roots in chemical and emotional problems that, once treated, disappeared.
It seems that when Christians try to pin stuff on Satan, medical science or technology or even basic knowledge later exposes the 'thing' to not be supernatural at all. A (conservative) Christian then has to argue either that "well, Satan might have manipulated your emotions or your chemical makeup to produce the depression" or "okay, maybe Satan wasn't involved in that, but he still totally tempts or even possesses people in these other kinds of situations..." Neither of which sounds convincing to me today. If Christians weren't already invested in a specific theology of Satan and felt compelled to defend it despite evidence to the contrary, would it really hold up under their own scrutiny?
More broadly, can Christianity stand on its own two feet without Satan as a major theological construct? What happens to the Christian story without an evil archenemy?