In contemporary Christianity, or at least my experience of it, several beings are generally considered to be the prime supernatural figure of evil, "Satan", such as the talking snake in the Garden of Eden, the being that argues with God about the moral character of Job, the being that tempts Jesus Christ in the desert, the source of the supernatural evil spirits Christ drives out, and the various monsters in the book of Revelation.
Were all these beings always meant to be the same, or were they later conflated? If so, how?
were they later conflated?
Yes.
If so, how?
Depends on the being in question!
If you'd like to read up on it, Elaine Pagels' The Origin of Satan isn't that bad, although you might have more fun with Prof. Philip Harland's Religions of the Ancient Mediterranean podcast - series 8 is a collection of lectures about the history of Satan.
Part of the conflation process takes place a century or so before the New Testament is written, when writings like The Book of Enoch start defining a cosmology that includes "fallen angels" with leaders who get human beings to do bad things. (And who are punished eternally in a place of endless burning.)
Part of it gradually develops as the writings we call The Bible transform from a collection of loosely related texts (some kind of folkloric, some histories, some poetry, some with elements of all of those) into a single text that, once bound in one cover, seems to call for a single way of reading.
So the talking snake, it seems likely, was originally just a snake - a figure in a story, like an Aesop fable. The Satan in Job was an angel tasked with challenging humans to prove their worthiness - ha-satan, the adversary. There's also the Morning Star in the greater prophets, which is a cipher for the king of Babylon, and the figure of Leviathan or Rahab, who seems to be a figure related to Tiamat, the Mesopotamian ocean goddess involved in the primordial battle that creates our world.
I'm trying to think if there's any exorcism verse in the gospels in which evil spirits state their origin, and I can't. Is there? Satan does enter Judas in Luke, but there's also the passage in Matthew/Mark in which Jesus calls Peter "Satan."
And Jesus is accused of casting out demons by the power of Beelzebub, but that's by other (ignorant) people. (Beelzebub in the gospels is probably a reference to the pagan god mentioned in 2 Kings.)
There are antichrists aplenty in the gospels and letters, but these seem to be early Christians who are doing it wrong, leading the faithful into bad beliefs or improper practices.
The Book of Revelation itself is kinda tricky because there's more than one satanic figure - usually read today as Satan and the Antichrist, a messenger of Satan, but at the time it was composed, maybe something else.
At any rate, nearly all of those figures have their own independent origins that gradually get wound into the lore of the figure we now call Satan.
If Harland and Pagels don't do enough for you, you can get plenty more on the Prince of Darkness by asking at /r/AcademicBiblical .
Follow-up: when did the concept become like an anti-God?
I'm not sure if this should be its own question, but what if we ask the same for Islam and "Iblis"?