Atheism versus Christianity shouldn't be too big an issue here. If you're going to properly examine the characters of Satan and God, or anyone in the Bible, you sort of have to stick to the source material whether you believe in it or not. You can obviously include outside but relevant sources, if appropriate. But if you're trying to argue the nature of God or Satan from a completely non-religious viewpoint, then no, none of it will ever make much sense. In the same way that Gandalf doesn't make much logical sense outside of Lord of the Rings, as a much more limited comparison.
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The Bible might be able to support many different claims now, but that's only if you try to apply the Bible to ways of life that never would have existed in the time and region it was written down in. Making a religious claim about a very modern problem is most often done by citing some line in the Bible that is so general it could apply to a number of things, or it's something that has been taken out of context in order to further a point. So yes, the Bible could be made to defend a number of conflicting viewpoints. However, when it comes to certain characters within the Bible, such as Satan or Michael, that's a somewhat different matter. Everything about their characters are limited to the book. It should theoretically hold up.
The only major rift in the Bible space-time-characterization continuum occurs in the shift between the Old Testament and the New Testament, which are separated by many years and a different culture. The storytelling is much different from a literary viewpoint. It's at that point that you have to determine which version is the one that holds the most 'truth.' Obviously, if you're a member of any religion stemming from Abraham, you're going to pick whichever one you follow. I don't believe in any, which might be useful or a detriment depending on your view of things. I apply the same reasoning to this matter as I do to other beliefs (namely mythology) that change over time.
The later versions might be more detailed and the 'classic' version, but it's usually the oldest one that is the purest, for lack of a better term. All the original intentions and themes are there, and those are things that can be lost with time (such as when Christianity came about, and especially when later Christian branches formed). Hecate is seen as a goddess of magic and the crossroads in the late Greek version of her stories, but if you dig deeper, you'll find out she was really just an Anatolian goddess adopted by the Greeks and given new themes to fit
their culture. If that can be used as a decent comparison to how I'm going about looking at Satan.
So with that explained, I look at the Jewish version as holding the most weight as far as characterization goes. In that family tree of religion, it's the oldest. Especially Genesis and the other earliest books. To me, God is as wrathful as he is loving. Satan just seems to be an evil toward
humans, not necessarily toward God. We're obviously going to think he's a force for all bad things in the world, but we're biased. He's probably just one of God's many agents, responsible for pushing humans toward their spiritual breaking point in order to test the strength and purity of their faith. When you transition into Christianity, you run into a few potholes.
1. Satan takes a turn for the very rebellious and malevolent, when before he just seemed to be a part of the grand scheme of things, if a cruel part.
-- 1a: However, to be fair, I would have to note the Second Book of Enoch. In 2 Enoch, Satan is lord of the Grigori, a branch of fallen angels that started screwing women left and right and fell from heaven due to their perverted ways and cruelty. Satan even got a little action from Eve, apparently, mostly because it seems as if he was angry at the creation of man and wanted some kind of revenge or attack. It's an apocryphal text, usually considered part of Jewish apocrypha. It came late, though, supposedly near the time of Christianity and from a sect of Judaism that wasn't widely acknowledged. In theory. It's possible that while the Jewish church tossed it out, some of the themes were then used for the infant Christian religion. Still, either way, it's a much later document than the early Jewish books, so it is subject to the same shifts in literary purity.
2. Dante's Inferno and Paradise Lost seemed to become so ingrained with western culture that their embellishments and fiction are sometimes considered truth. I've seen Christians take elements from both stories and try to pass them off as actually being part of the Bible, even when the text doesn't really seem to support some of the claims. It's possible that such writings have influenced things in a way they shouldn't have. The war in heaven is especially played up, considering the biggest source of it is from Revelations, which takes place at the end of all things, not early on. The fire and brimstone days of the Catholic church may also be of some less than accurate influence on matters.
Regardless, things get played up to a level they weren't on before, and certainly not in the oldest texts. Either way, God, being omnipotent and whatnot, could have easily gotten rid of him. He did so in Revelations, which seemed like a giant ass whupping and not a lengthy struggle between two equal forces. So it's very possible that while Satan may have considered himself an outcast or rebel, God continued to use him as a chess piece of sorts. Or maybe Satan was willing to take on the guise of being the bad guy. Or maybe it's just humans who paint him as evil, considering the nature of his job.
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As for God being cruel, I'm with Steel in that you just have to deal with it and accept it as part of the religion. God may or may not have good intentions about it. That doesn't matter much either way. And if you take Judaism into regional and historical context, God being just as destructive as he is loving makes
absolutely perfect sense. Almost every leading god in that region and time period was capable of bestowing great gifts upon humanity... but was also capable of massive destruction and plagues. And they made no apology for it. In the historical context, God being cruel fits in with the themes going on during that era. But that's from a more distant viewpoint on multiple religions. May not be appealing to people now, but that's the way it was. And God only called one Deluge. Enlil called a lot of them! So the Old Testament isn't even necessarily the most destructive, as far as they all go.
God being JUST love seems to be a product of megachurch pop preaching, a way for salesmen pastors to attract newcomers. Because he doesn't come across as being only love and goodness in the texts.
Sorry. I think I rambled there for a while.