Not entirely arbitrary, given that almost the same thing happened in England. Rather that the authority of the Church was cracking, and both popular movements and rulers took advantage of it to grab a bit of the Pope's power. But certainly it was more to do with politics than theology.Zoroaster seems to have been the inventor of the idea of a power of Absolute Evil (as distinct from a god who might harm you because he was another tribe's god).
I am no expert on satanism, but I do know it is the Christians that create a devil who hates people.I think it's generally accepted now that the Church invented satanism. Executive summary: when the great heresy persecutions of the 13th Century were tailing off because they had run out of people to burn, some Spanish monks who were struggling to convert stubbornly pagan peasants in the mountains had the brilliant idea of rebranding their unglamorous work as another campaign against heresy. They announced that they had discovered the ultimate heresy, people who actually worshipped the Devil, and the Church took it up with a will and started hunting for Satanists everywhere. And as Pratchett observed, the thing about plots and conspiracies is, even if there aren't any to start with, if you hunt for them vigorously enough, pretty soon there will be. Once the Church (and the new Protestant churches) started telling everyone how terribly powerful Satan was and how he tempted his followers with rich rewards, naturally people started to think they'd like a piece of that.The little I now about satanism is that it is actually many things, that some of it was established as a protest against and attempt to liberation from the many restrains Christianity places on people, that some of it is sort of nature orientated, and that some is some sort of magical universe.Yes, it only appears in the book of Lot, where he seems to be something like JHVH's official torturer. Probably originally refered to an official similar to a Public Prosecutor.The word 'satan' is apparently Hebrew for 'opponent' or 'accuser'.