Egregoros

There were no "gnostics" though. No one considered themselves as such. It's a retroactive term for certain groups that died out. Their grouped together, but they're all very different. A Valentinian isn't going to have the same beliefs as a Manichaeist. A lot of the groups grouped together as "gnostic" didn't even believe in the monad/demiurge.

Some are simply placed in the classification because they made have had versions of the Gospel of Thomas or the Gospel of Mary. Some of these texts disappeared because they just weren't popular and priests stooped copying them, others were burned as heresy.

Christianity isn't a straight line, it's many many different groups that came and went, fighting for survival, until solidified by Constantine (in the same way ibn Marwan solidified Islam under the Mohamed story as a foundation for his rule in the Caliphate).

Satan also just meant "adversary" (or possibly even "advocate" in some places). There are several places where it's "the adversary" or "an adversary" and the translations are a bit dubious. The oldest book of the Bible is Job, and the NIV at least has the right footnotes, showing the story is about a greater pantheon of gods of which Yah was a member. Earlier proto-yahweh beliefs indicate he had a female consort at some point, Asherah. By the time of the Bible we now know, this was believed as heresy and "Asherah poles" were mentioned as something to be destroyed .. but it's quite obvious these two believe systems existed at the same time for several generations.

The idea of Satan being an entity didn't seem to appear until 500 BC, and likely came from Zoroastrianism, influenced by the Judean people living in the Persian/Achaemenid Empire (although there's no direct evidence). Before that .. the concept doesn't even seem to exist.

The idea of "Lucifer / light bringer" comes way later (Lucifer is Latin of course), and that's where it gets really messy tracking down where this particular idea originated or the very weird reinterpretation Isiah 14 to use a few short versus to invent entirely new lore!

Most Christian beliefs today are just retroactive cannon. They obviously believed in other gods ("You shall have no other gods before me), but Christians interpret this as other gods being demons and fallen angels (something people who followed Abrahamic teachings likely didn't believe at the time).

It's all really a lot more complex than people realize.