Ok some thoughts:
This is very wrongheaded and bizarre, and leads me to believe you either read a lot of information faster than you could understand it or made shit up to fill in the gaps of what you didn't know. Or you might have read some gormless AI search result to do both of those for you, who knows at this point. Now, I don't know a lot, but I know just enough about this to know your shit's ALL fucked up. Like, holy shit man. It's so intellectually annoying that I kind of couldn't think of anything else outside of writing this post, I have to untangle this. I have a problem.
For one, we don't have the Iliad because it was all written down in stone somewhere like the fuckin' Xi'an Stele, we have it because people made several, several copies of it (on papyrus, mind you) and some of those made it to the modern era. Writing on stone is expensive as hell in terms of time and resources, it limited the information you could write down to just the size of the flattest part of the stone, and it certainly didn't travel well. Clay tablets and cylinders did preserve a little better than papyrus generally, but we have more papyrus and other paper types because you can make more copies that will, by random chance, survive.
For another thing, the closest thing there is to a "default" stance in the bronze-age Mediterranean is a position called Henotheism, which acknowledges some level of reality for gods outside the general purview of the ones the individual worshipped. There are aspects of this still practiced today in the large web of Dharmic religions, where a lot of them only revere one or a few gods but acknowledge that others are possible and might be more valid for other people. I don't know what you mean by "there was a tendency for monotheist religions to wipe each other out" as if it's some sociological/doctrinal thing, but that's, like...
Okay, I realize there's so many blatant fuckeries here with the mechanics of how these things played out, that I've actually started in the wrong place to begin to explain them. Like, enough that it feels like I'm reading a historical account by somebody whose main interaction with historical content was Civ 6,and I've started out my explanation by saying that the colonization of India did not happen because Catherine De Medici and Elizabeth I declared war on Gandhi. Like, that's true, but it isn't addressing the underlying problems here which is bad if you want to establish an understanding not of that individual event but of the systems that brought it about in general. And that's what this kid is after, learning about the ancient gods, all of them.
Religion is generally theorized to have started out not as polytheism, but a much broader umbrella of practices called "shamanism", but this is kind of a loaded term that has a lot of scholarly debate around it due to the way it's been used historically by religions with centralized political authority to kind of root out and imperialize religions that don't. There are many diverse practices that fall under shamanism but for the purposes of creating a narrative to understand how one thing might become what we're currently talking about, there is less of a hierarchical understanding of capital g gods in the sense of immortal creators or embodiments of things, and more of an interest in "animism", or, the anthropomorphic assignment of personhood, will, and intent to natural forces. Fire is an easy one for modern people to understand because so much poetry and fantasy writing still treats fire like this. But there's also, like, other belief systems and practices that have some commonalities-- Ancestor spirits, spirits belonging to places and inanimate objects, belief in a sort of personal system of magic that can be practiced to ward off misfortunes or bring about good things. This isn't a cut and dry category, not all religions that are called Shamanism have this, and, you will find, studying the practices of polytheist and even monotheist religions that follow after, they are not devoid of these things either. (In fact it could be argued that in actual practice, norse pagans were closer to practitioners of "shamanism" as previously described than they were to "polytheism" in the sense that ancient greeks practiced it. But it's all gray areas here because scholarly language is meant to describe, not prescribe, what a thing is, and every individual believer is different, let alone the way every belief system can function in practice.) But this is an illustrative model you you can kind of see, in an iterative system of storytelling and ecosystem of diverse beliefs, how these might spread and change into other things over time.
Ancient Egypt is a good example of this, in a sense, because the religion and its practices feature a lot of these precursor beliefs, namely a kind of animism, everything having its own spirit, personal magic woven into everyday life, and sometimes appeasing ancestors or petitioning them for help. Ancient Egyptian religion was highly syncretic, and had a niche god for just about everything. The world of physics itself was, in fact, a system of gods who made everything work, in the mind of an ancient Egyptian. A lot is made about Anubis's role as a psychopomp and a tomb guardian, because to the modern mind making him a kind of Ancient Egyptian Grim Reaper is more evocative and gives him a kind of wider-reaching role, but it should be emphasized that he was also the god of the mummification process itself, and back then, that was considered equally if not more significant. The steps of mummification were not understood as merely a sequence of chemical reactions (Or, I guess they kind of were, the roots of what we consider to be alchemy pretty much started in Egypt, but I don't want to be here literally all day so let's move on from that) but like, as something that happens because you are working with Anubis. He is the process, it isn't sodium chloride drawing h2o molecules out of the body, they didn't have microscopes or a table of elements. A lot of the gods of nature, civilization, and individual households were like this, but then you get into the idea of the major, "head gods", and you get something more akin to what other civilizations were primarily doing, which was the state religion.
I'm going to be EXTREMELY broad here to save myself hours of describing edge cases and compare-contrast here and say, if you want to get a lot of things wrong but still be mostly right, the "meta", by and large, for bronze age citystate-style civilizations, was this: You would have a god, a big god, usually related to objects in the sky or the weather, and he would represent your area, your place where you lived. They didn't have an idea of a nation as this secular intellectual idea, you were a person from Ur, who belonged to the city, if you worshipped Nanna-Suen, the Moon, and participated in his rites and holidays, which were major economic and social events for the city. That's like, a large part of what made the system work. And furthermore, that god would also typically have a spouse and children to denote not only relationships (usually of subordination or inter-reliance) between other cities or important industries and resources, but also so that that divine bloodline could be traced down to a human who would be the king or queen of that place and exercise authority over it by divine right.
So with that understood, now we're finally ready to actually address that point you made. In most cases, if a person venerated only one god, it was most likely either the god of their tribe or citystate that they would have been socially if not legally obligated to worship. If that person's culture was destroyed, moved to a different place, or otherwise just changed in a significant enough way, that god would also most likely change. But it wouldn't always simply be lost. Very often it was a propaganda victory for the oppressor if they could make the god of another place subordinate to theirs, or indicate that for some reason the god of another region was angry at its people. It also need not strictly be negative, it could be the result of an alliance or large shared influence. Sometimes they change names to reflect a shift in language, sometimes they combine with other gods when two cultures notice similarities between them. The history of mythology is full of stories where the characters are different between different areas. If you've been on the internet long enough, you're probably aware of many cultures having a "flood myth", and only in one case is this attributed to the god of Abraham.
Like, most of us probably don't imagine Artemis, goddess of the hunt and single ladies, as the patron deity of Ephesus. But she was, and the fact that people came to her for all kinds of things, not just stuff strictly related to hunting or archery, that led to a lot of people coming from all over Greece to her temple in Ephesus, which was one of the 7 wonders of the ancient world and gave the place a ton of money and clout in the Hellenistic world. In a lot of cases where individual cults or religious practitioners who revered one god above all the others didn't make it through history, their god would survive as an addition to part of the rest of mythology. And that's the part we actually don't know. Because we don't know if that's actually the case with Artemis, only that she's very different in Ephesus than she is in other parts of Greece that recognized the Olympians, and she's the primary goddess of that place. We don't know if they started as "monotheist" (at least, to the extent that monotheism was ever officially practiced in that time). The origins of the gods are not recorded, we do not have the first drafts of the myths, we only know the earliest things that were written down AND survived, and we never exactly got to see and record the differences in every cult of the gods who ever practiced.
Speaking of the Greek Gods, that aforementioned state deity thing where the lineage of royalty to the gods is their justification for rule, is sort of what the Greco-Roman gods were doing. It's also a big part of why they're famously so incestuous, because back in the day when the ancient Greek gods actually were supplanting (or, more likely, co-mingling, but we're getting into that) with other religions and individual practices, it was cool to add them to your family tree by making them the ancestors of your local heroes and then being descended from those heroes, who are very often kings and queens and other royalty themselves. And at that point you're drawing from a pool of 12, maybe 20 gods, and you pick the ones that might make symbolic sense, but nobody wants to think they're only following the king who's descended from a chump god who's lower on the totem pole, so naturally Zeus or Poseidon have to be involved at least at some point in the arrangement, but also those guys are close blood relatives to all those people too. But that's merely a funny side note, I forgot why I needed to delve further into the Greek gods here, let me read the post again.
Oh, right, Yahweh. Goddamn, the thing that bothered me from the start. I spent a long time googling trying to find where you even found this claim and even the sketchy websites weren't turning up anything. It's actually batshit insane. For entertainment's sake alone I would actually like to hear what deranged source you have on this shit, or at least hear the debates you managed to win making this point, because that isn't even the most atheistic interpretation of current scholarly consensus. This doesn't even stand up to very basic scrutiny because the trinitarian idea of the Holy Spirit as an agential being in itself is a heavily Christian, trinitarian interpretation, and the Jewish concept of the Holy Spirit is something entirely different, and it's used as a term to denote the force of god causing things to happen, more akin to saying "Hoganly Resilience" if you were describing the supernatural durability of Hulk Hogan. ('Twas the Hoganly Resilience that moved the Super Twin to no-sell the Tombstone at WrestleMania, not the weird ego problems of a dude who was going to WCW next year anyway) the idea that it could be a semi-separate entity in itself is an interpretation put upon the text, it is not explicitly treated this way in the bible. There is no surviving evidence that this "Holy Spirit" is the fossilized remnant of a pagan god that's fallen out of popularity-- And we know this, because the bible has fossilized remnants of pagan gods that have fallen out of popularity, and we know what to look for.
The historical and archeological evidence we have, would suggest that Yahweh was a god of storms and river flooding, who came to represent an individual tribe that became part of the Israelite cultural complex. If I'm really stretching what I think you might have misread or been trying to say to its absolute limit, there was a point where, when Israel was part of the Canaanite sphere of influence, Yahweh the storm/war god was viewed as subordinate to El, a kind of elder/king figure in Canaanite mythology, but over time they seemed to merge into one identity due to perceived shared features between them, and convenience of treatment. As previously mentioned, this isn't an unusual thing to happen over time in this religious landscape, look up statues of Hermanubis if you'd like to see a particularly interesting example of this. The idea of Yahweh actually goes through several very interesting transcendances like this over the course of the bible as Israel's situation changes.
It's a very long book and I don't remember everything exactly in order, I'm just going in order of what makes this easy to explain, but the bible begins to implement a number of rejections of the ways other primary deities functioned in response to things the Israelites were dealing with at the time. Much of the very early bible is Henotheistic, and the earliest statements about God having no other are less about the rejection of the existence of other gods, and more, like... Well, for one thing, he's supposed to be the head god, in charge of Israel, and revered by Israelites above all others. But the other thing is actually quite interesting-- God has no wife. You might think that sounds weird, like, of course, if you're a monotheist your god probably doesn't have a wife. But the thing is, if you were the kind of guy that only went to efforts to revere one of your gods, your state god, even then you'd probably acknowledge that your civic god had a wife, because how else would he have descendants? And if your god didn't have descendants, where did your king come from? This is why the bible has rules against "Making an Asherah", and all manner of other idols associated with this sort of deal. Like, it was considered normative practice at this time to assume that your god, even if you only cared about the one, had some kind of family unit, but the God of Abraham was and is, I daresay, something queerer, operating outside the bounds of human reproduction.
The royal line of Israel is not that way because they were infallible superhumans literally descended from the MIGHTY SUN, who would all unilaterally belong on the throne no matter what, but because they were directly chosen by god, and we are all equally "children of god" in the sense that we are descendants of Adam and Eve, who he made. The religion of the Israelites would be shaken multiple times in their history in ways that would drive them in ways to further separate their idea of God from being, like, tied down to a specific place and a specific people the way other gods were. There are predecessors of this line of thought in previous points in Jewish history, and even in some neighboring mythologies, but because I don't know enough to feel confident talking about it I'll say this really crystallized when Rome destroyed the second temple, and economic circumstances (and a bit of slavery) wound up scattering the Jewish people all over the empire. Very suddenly, the holiness of God couldn't just be tied to a place, because that place was gone, and there were a lot of Jews who could not return there anyway. It couldn't be tied to a kingdom, because that kingdom was vassal to an empire that had just committed probably the biggest blasphemy that it was possible for a state to commit. This led to a fascinating period in theology called "apocalyptic Judaism" which I'm not even going to bother getting into here. Probably the most famous figure from this period was a guy they call Jesus-- Some of the conclusions that he and theologians after him came to about the nature of a singular monotheist god should be familiar to anyone reading this post. We gotta move on from this eventually though, and since this is a discussion about mythology I can literally say I've made the mistake of starting parts of my explanation at the fucking beginning of time. I need to be quicker about the next thing.
These next two things are wrong in a more abstract way, the first is just a common phrasing of things that annoys me, that the romans "copied" the gods which, like, I know few people ever actually say that genuinely, but it's still fairly annoying. The Romans simply had the same religious dialect as other dudes on the north coast of the Mediterranean. They believed in the 12 olympians-- Not that they all had thrones at the top of Mt Olympus, but that was not a universally held belief by everyone who believed in the gods we call the "Olympians" either. In fact, you'd be hard-pressed to find a kind of universally held belief between them! Because here's the thing about "polytheism", or really, how all religion worked in ancient times. It is, very distinctly, not Christianity!
There wasn't a pope, there wasn't an administrative oversight committee of highly educated book nerds all collaborating to make sure everyone has an agreed-upon set of stories and names that are all kept straight. In the age of citystates, you had temples and holy sites that were tended to by a priesthood, and those priests more or less were part of the nationstate of that city. They might have had hierarchy or done some oversight to keep the story straight within their own area, but the "rules" of that religion were handled, from a practical perspective, more or less at the civic level. If somebody declared themselves the Pope of Zeusism*, they might be able to get away with it, within their city, but it would be that city's Zeus. The cultural network around which these gods were recognized spanned a very wide region, across numerous cultures, languages, time periods, and styles of government. You might think it's funny that the gods of Rome are like the same guys with different names, I think it's funny that these different-named guys managed to be so similar!
The truth of the matter is, there is no singular "Zeus". A lot of cults in the Greco-Roman world had a guy like Zeus, and the religious concept of Zeus gradually developed as religious institutions in different citystates communicated, and other travelers recognized commonalities in the beliefs, stories, and practices of different places. But just like DC has Superman and Marvel has Wonderman, different interpretations of the same shit can wind up being really different!
Like, take Mars and Ares for example, they're characterized very differently because they are meant to communicate different things, by two societies who valued different things. All the "Olympians", or "Dii Consentes" from the Roman perspective, have little differences between their Greek and Roman counterparts. Some stories fall out of favor or have details changed. But Mars and Ares are a good case study because they are the two most clearly different versions of one of the big 12, you can tell from their statues, you can tell from the shift in how people talk about them. They are practically different characters, though their domain and role in some of the stories is the same.
Ares in the stories that feature him is stubborn, bestial, brutal, barbaric, full of false bravado, and morally wretched. He is still a powerful archetype to be revered, but he is the kind of spirit of war that is useful to the soldier but not to the general, embodying the pure act of fighting other humans. This is treated as having a very limited place in Greek society. Often stories serve to humiliate Ares or highlight his repugnance in comparison to Athena, who represented a kind of civilized organizational view of war that was viewed as preferable. Mars could hardly be a more different depiction-- He is, to use a very anachronistic term here, chivalrous, and represents courage as well as a sort of idealized conduct. Roman society placed great value on the concept of a virtue they called "Virtus" (look it up, I've been at this too long as it is.) which was heavily associated with the conduct expected of ideal military men, and while Virtus was literally embodied by a goddess named after the concept, Mars was meant to possess virtus in spades. Men in Rome were expected to acquire virtus by going to war, and nobody has been in more war than Mars! That would maybe have sent a weird message if they went with the other interpretation.
Anyway, the other point, and the point on the polytheism end of things that strikes me as the most obnoxiously ignorant, was the "The Ancient Greeks were aware their religion replaced an older religion, 'The Titans', which were perhaps the oldest religion we know of". Now, if you've been reading along here hopefully you've had enough concepts established about how the Greco-Roman Religious Complex "worked" that you can already see how shaky the ground is for this statement.
But let me be very clear, "The Titans" do not comprise the oldest religion we know of. For one thing, if the bar is as low as "we know of it", then anything we have names and written stories for is probably not on the first page of the oldest religions we know, because then we're competing with the likes of stonehenge and paleolithic Shaktiism. For another, there is no singular start date for just about anything in the Greek Myths**. Individual gods may be way older or way younger, we don't know exactly where a lot of these things came from, we merely record when they appear in data that survives. They are also, for most intents and purposes, the same religion as the rest of the Greek Gods. Athens had a temple to Cronos, he was considered as valid a deity for reverence as any other, and was worshipped at the same time as the other Greek Gods. The Titans and The Olympians are the same species of being, the difference is more like a generational social class-- There was a time when Cronus's generation of gods ruled the heavens, and now Zeus's does. That's the difference. If you want to get especially trippy, it's possible that the concept of the current holy family having overthrown a previous generation of gods goes back before the Greeks, but rather to the Hittites, and became a shared/adapted story in Greece. If indeed new gods replaced any old ones, it was somebody else entirely. Much like in the Bible, there is Greek mythological evidence, which, paired with archeological evidence, can be used to deduce the case where the Olympian cults actually did gradually replace an older religion, and it is not the Titans. An example of this would actually be the story of the Minotaur, which would seem to contain references to an ancient bull cult which we know existed in Crete before they started worshipping gods more familiar to the rest of the Grecosphere-- But also establishes that the royal family of Minos are descended from multiple Olympians, who got involved with multiple generations of King Minos' family. (Yuck!)
*This is a stupid thing for a person to do, for a number of reasons, because that game was locked down tight by dangerous people. Folks who were going into business for themselves, religiously speaking, tended to form cults for local heroes or city patron deities where their influence wasn't entangled with the voices of everyone else in the greco-roman world who would certainly have an opinion on it. Though if you were more philosophically inclined, you could yourself become a kind of guru-- I also don't want to take too long here at this point because the post isn't even done and I'm writing ADHD footnotes, but if you would like to go down a funny rabbithole, let it be known: that all that triangle shit? Absolutely the least interesting thing about Pythagoras. If you're still in school be sure to read up on the guy and let your math teacher know what a psycho that guy was.
**Unless, of course, we have reason to believe one of the stories or interpretations of these stories which went on to become very popular was original content written by a guy in the form of a play or a parable or whathaveyou.
I don't know how to end this, I refuted what I wanted to refute, said more than I wanted to say, and now it's midnight. Fuck you. Fuck off. Go home. I'm sleeping. If you've ever heard it said in a sunday school class or history class or whathaveyou, that before Christianity, Rome generally didn't care what you believed as long as you paid taxes? This is why. All this shit. And of course this comes with the standard disclaimer that this has all been rattled off the top of my head out of spite, so UD please make sure you actually read things written by real people with real labcoats. Don't listen to me, but ESPECIALLY DO NOT LISTEN TO THIS GORMLESS FREAK.
