Let us say polytheistic Hebrews/Isrealites in place of polytheistic Judaism.

To know the origins of Hebrew polythiesm, we must understand the origins of their religious traditions and gods, the ethnic/genetic origin of the Hebrews, their ancestors and kin, and all such in order to give a clear answer to your questions. However, I will focus overwhelmingly on its origins and how it relates to then-contemporary and even present existing Afroasiatic religious traditions, although I will my give some thoughts to what may have caused the shift of identity for Yhwh from being a lesser God to being tied with 'El' and becoming the national Israelite God towards the end.

The Hebrews are a Semitic ethnolinguistic group, which means that in common with all present and extinct Semitic groups like the Arabs, Assyrians, Babylonians, Pheonicians, Sabeans, Amhara, and the Tigre (and numerous unmentioned others), they also speak a language that descends from the proto-Semitic language, which was spoken about 7-4000 ybp. The proto-Semitic language was spoken by the proto-Semites, a people whose descendants include the Hebrews and the various other mentioned - and unmentioned - Semitic groups.

The proto-Semites first arose in the area which is now termed the Levant, which includes Syria, Lebanon, Isreal, and Jordan. The proto-Semites are believed in turn to be descended mostly from an indigenous Levantine mideastern population, and a small migrant Afroasiatic population from northeast Africa who mixed and accultrated the indigenous Levantine population, who then shifted from what pre-Afroasiatic language they were speaking to the Afroasiatic language of the African migrants. The resulting population from this nominallly African-admixed Levantine and now Afroasiatic speaking population became the ancestral proto-Semitic peoples, or Natufians.

Now when the Afroasiatic migrants accultrated the Levantine population, they didn't simply induce a language shift, but they also introduced the Afroasiatic religious traditions they brought with them from Africa. To know more of these traditions, let us speak on the origins of the ancestors of the proto-Semitic peoples and their Afroasiatic ancestors, the proto-Afroasiatic peoples.

The present scholarly and academic consensus agrees that the cradle and homeland of the proto-Afroasiatic peoples and language to be in the Horn of Africa (Ethiopia, Somalia, Eritrea) and immediate vincity (northeastern Sudan), first arising prior to 15,000 ybp. The earliest proto-Afroasiatic people were hunter-gatherers at first, and they are likely to later have became arguably the first to harvest wild grains (which in turn, lead to early agriculture). They later domesticated various animals, including cattle, camels, goats, and the like, and became mostly nomadic. The proto-Afroasiatic people are believed to have had a henothiestic pagan religion. Since proto-Afroasiatic society was centered wholly around clans, the henothiestic Afroasiatic faith centered on the worship of the God of the clan, with each clan having a ritual clan-priest (*waper in proto-Central/South/East Cushitic). The *waper clan-priest served as an intermediary between the clan-god and the clan, and played perhaps the most vital religous role in proto-Afroasiatic society. - The henothiestic Afroasiatic faith also had a high central all-powerful sky-God figure who was superior to the lower set of clan-gods. This high sky-God was the creator, but the exact role of the high sky-God and influence in the lives of the common people is not clear as of yet. This high sky-God is present in all derived Afroasiatic religious traditions, as the 'Waak' of the Cushitic peoples, the 'El' of the Hebrews/Israelites/Canaanites, and 'Allah' for the Arabs (Allah and Eloah being cognates).

In addition the to high God and clan-gods, the proto-Afroasiatic religion was also dualistic, with a good vs evil dualism. They believed in the existence of demonic spirits/beings of a cursed and evil nature, and these spirits came to later manifest themselves in the derived Afroasiatic religious traditions, called 'saar' or 'jinni' amongst the Cushitic groups, and 'jinn' amongst the Arabs and other Semitic groups. In addition to the evil spirits/demons, they also believed in angelic beings and benevolent spirits sent by the high God to assist the humans. These spirits later became the angels of the Abrahamic tradition ('Malak' in Arabic and Hebrew), and the Ayaanle of the Somali and other Cushitic peoples (or 'Ayanna' in Oromo).

And then there was the practice of the the now dominant Abrahamic tradition of circumcision, which also first arose amongst the proto-Afroasiatic people as a socio-religious rite. I go into an in depth explanation of this in this post on the origins of circumcision:

Https://m.reddit.com/r/AskHistorians/comments/4ixj8i/how_long_ago_and_why_did_humans_start_practicing/d32i2ve

*To summarize, circumcision - for both males and females - was of high importance to the proto-Afroasiatic peoples, and remained greatly preserved and intact in most of the descendant Afroasiatic cultures and peoples of all branches, being notably so in our current Abrahamic (and Western) tradition of circumcision.

The proto-Afroasiatic peoples also practiced some form of dietary restrictions, and this early tradition became preserved in later Afroasiatic religious traditions as 'halal', 'kosher', and the Cushitic 'fish taboo'.

And then finally, there was the proto-Afroasiatic tradition of animal sacrifice to a clan-god and/or high God. The tradition of animal sacrifce became a cornerstone in all derived Afroasiatic religious traditions, both pagan and Abrahamic. It is also likely that the drinking of camel and cattle blood, as was practiced by the Cushitic peoples until of recent (and by Cushtic-influenced groups like the Masai), was also a likely proto-Afroasiatic custom.

As you can see, minus the polytheistic nature of the religion, almost all features present in the Abrahamic faiths were all simply relict proto-Afroasiatic rites and religious traditions, with a creator high God, angels, devils, curses and possession (which I did not get into), priests, animal sarifice, dietary restrictions, and cicumcision. As such, let us return to the polytheistic Hebrews/Israelites.

So, when the proto-Afroasiatic and Afroasiatic people first began experiencing population growth as a result of food abundance and putting pressure on the neighboring Nilo-Saharan peoples to their direct west, and the Khoesan hunter-gatherers to their south, they began successive migrations north, south, and west. The Afroasiatic migrants and clans that went north followed the Nile, and therein it is believed they then broke into 3 groups, one heading west and becoming the proto-Tamazight/Berbers, one staying put and becoming the proto-Egyptians, and a third group which crossed the Sinai and into Asia and the Middleast, finding an already settled population of Levantine people whom they admixed with and accultrated. These early ancestral Semitic peoples are the Natufians, whose genetic makeup revealed a heavy northeast African influence (30% avg.?), as well as exhibiting clear Afroasiatic influence, with the excavation of what is believed to have been a Natufian *waper clan-priestess in the tradition of her northeast African proto-Afroasiatic forebears.

The descendants of the Natufians/ancestral Semites were the proto-Semitic peoples, whose descendants the Hebrew/Israelites number amongst. The proto-Semitic people had developed new unique features within their Afroasiatic religious tradition, many likely as a result of substrate influences similar to that between the Semitic Akkadians and the non-Semitic Sumerian substrate (Sargon, etc.,). As such, Semitic Afroasiatic henothiesm most likely significantly or slightly differed from that of their Afroasiatic neighbors and kin on the African mainland.

With this in mind, it seems that by the time the proto-Semitic peoples had migrated and diversified all over the Middle East, Levant, and Mesopatamia, the common Semitic pagan tradition of the various Semitic groups began to differ significantly. However, almost all kept most of the proto-Afroasiatic traditions alive, and all most likely adhered to a locally differentiated form of the common Semitic pagan core pantheon.

The core Semitic pantheon had developed prior to the time of the proto-Semitic migrations during pre-diversification period. This core Semitic pantheon most likely arose exactly like that of the ancient Egyptian pantheon, with the various proto-Semitic individual clan-gods each later becoming a God in their own right and getting their own unique association. Thus, the proto-Semitic clan-gods coalesced into the core Semitic pantheon, with the proto-Afroasiatic high sky-God being preserved as the central creator and main diety, and presiding over the pantheon.

All the Semitic peoples worshipped the core Semitic pantheon, including the Arabs, Sabeans, Syriacs, Ge'ez, and Hebrews/Israelites/Canaanites. However, each also differentiated the pantheon by the addition of other gods or deletion of others, likely depending on their usefulness, with an agricultural and settled peoples probably not having much use for a desert God, and thus, having the desert God fade into complete obscurity as time goes by gradually.

(Continued in 2nd post)

*To whoever gave me gold, I am greatly humbled and appreciative.

*Folks, I put some sources down below in my 2nd post. I'll be adding whatever else I can. There is a significant amount of recent linguistic and archeogenitics research on alot of the early prehistory I mention here, and the evidence is currently undisputed for the most part. I'll compile what else I can.

(Continued from 1st post)

So the Hebrew/Israelites adhered to the core Semitic pantheon, with the Afroasiatic central high sky-God becoming 'Yhwh', and 'Elohim' being a plural, meaning Gods', i.e the Israelite pantheon. The word Elohim is cognate with the similarly sounding Arabic 'Ilaah' or 'Gods'. However, both words can also be used in a singular form.

However, it is possible that the Hebrew/Israelites were influenced by the neighboring and fellow Afroasiatic ancient Egyptians, whose theology and religious scripture probably had a role to play in influencing the religious traditions and beliefs of the Hebrews and thus, the Jews of today.

One important thing to note is that the main sky-God of the Semitic pantheon was refered to as 'Il'. 'Il' became the 'Eloah' and its plural 'Elohim' of the Hebrews, 'lah' of the Arabs ('Al-lah', 'the God'), 'Ilu' of the Assyrians, and so on. However, then it begs the question as to how 'Yhwh' came to be used to exclusively refer to El.

'Yhwh' first began as one of the 70 sons of 'El' the high sky-God of the Israelites. Each of the 70 ons of El ruled over a nation of peoples, and 'Yhwh' alloted nation was a part of Israel. Probably because of this, his name became increasingly associated with the main sky-God El in time, and later came to exclusively to refer to the sky-God and replace the former name.

Later on in time, perhaps the Israelites came to see 'Yhwh' as embodying the powers of the whole pantheon of gods. Or perhaps due to the belief the sky-God El's sons each being the god of a nation of their own, it is also proposed that perhaps the defeat inflicted by the Assyrians really shook Isrealite society and it's religious outlook, and with the Assyrians being the most dominant and powerful polity at the time, then it must have meant that their God must be just as powerful as well. As such, the god-nation divine pairing became unfavorable to the defeated Israelites, with the central sky-God 'Yhwh' coming to be seen as the only God of all nations, a much more favourable religious concept than that of the former, which would have given an image of divine unbreakable military superiority to the Assyrians.

However, a better theory not yet mentioned here is the contemporaneous and symbiotic melding of Israelite society from a pluralistic, decentralized state and existence, to a highly centralized, singular state and existence. What is being referred to here is the Davidic monarchy and the establishment of Israelite monotheism and a singular Yahweh which progressively developed from a decentralized and polytheistic background. Both state and religion seem to have an uncanny similarly evolution towards singularity, and thus may have reinforced each other as such.

And interestingly, I have an explanation for how the state and religion both seemed to move together, almost lockstep, towards a singular form:

The Israelites, as I have mentioned, were undoubtedly henothiestic, and had preserved the worship of clan-dieties and the office of clan-priest *waper. So we can see a direct connection between clans and polythiesm/henothiesm. As such and as we usually predict, clan-systems are most vibrant in decentralized sociopolitical regions and modes of organization. Thus, we can expect that the opposite of decentralization -centralization - to have the opposite effect in that it is detrimental to any clan system and sociopolitical form of organization (as witnessed in Ireland, Scandinavia, Southern Africa, China). Thus, with the shift from a clan-based, decentralized system to a highly centralized monarchy broke the power of the clans, and greatly lowered the importance of clan affiliation amongst the Israelite populace. With the lowered significance and importance of the clan system, and the rise of nationhood wherein one would identify themselves as a Hebrew or Israelite as opposed to their clan, Israelite polythiesm and henothiesm, with its direct reliance on clan-priest and clans, died off. As such, the shift to a more common and broad singular identity as opposed to the numerous clan-based identities, served to excommunicate the vital role the clan played in the Israeli henothiestic and polythiesm faith from Isrealite society, and Israelite monotheism possibly being a manifestation of the more singular identity of the nation which united the various Israelite clan identities to form a whole, just as happened with the melding of the Israelite pantheon into a single God figure.

*To say 'Elohim'/'Eloah' and 'Ilaah'/'Allah' to mean 'Gods'/'God' is far too simplistic and undeserving, and the true meaning of those two word is far more complex, and as such, the English language lacks any true equivalent word, although 'Lord' seems to be much preferred in translated religious texts.

Kitchen, A., C. Ehret, S. Assefa, and C. J. Mulligan. "Bayesian Phylogenetic Analysis of Semitic Languages Identifies an Early Bronze Age Origin of Semitic in the Near East."Proceedings of the Royal Society B: Biological Sciences 276.1668 (2009): 2703-710. Web.

Halladay, Andrew, "The Ascension of Yahweh: The Origins and Development of Israelite Monotheism from the Afrasan to Josiah" (2010). Pomona Senior Theses. Paper 5. http://scholarship.claremont.edu/pomona_theses/5

Laichas, Tom. "World History Connected | Vol. 2 No. 1 | "A Conversation with Christopher Ehret"" World History Connected | Vol. 2 No. 1 | Christopher Ehret: "Christopher Ehret" University of Illinois, 2003. Web. June 2016. http://worldhistoryconnected.press.illinois.edu/2.1/ehret.html.

Ehret, Christopher. The Civilizations of Africa: A History to 1800. Charlottesville: U of Virginia, 2002. Print

I might be able to answer this for you using evidence from the site of Kuntillet 'Ajrud. However, I would like to point out that the Israelite history is very in depth, so one post won’t cover much.

Briefly speaking, we don't have concrete evidence of where the Israelites originate from, we have a few theories regarding their appearance in Canaan, but there is no material evidence to 'anchor' a date or an origin (you can google these theories by Albright & Yadinl; Alt & Noth; Mendenhall & Gottwal; Finklestein). The other issue we have is that during the early formative period of this history (appx 8th century BCE), writing regarding Yahweh was very limited, therefore we don't have many written sources attesting to Yahweh's role in the pantheon or a real theogony for him.

So if we assume that the Israelite people, before they assumed an Israelite identity, were integrated with Canaanites or native to Canaan then they would have had influences coming from the Near East (Assyria/Babylonia etc) and Egypt to say they least. Additionally, the period of time which you are referring to looks at between 800 BC - 550 BC which is the Iron IIB-C through to the second temple period. Yahweh's role in the Canaanite pantheon was originally that of a minor deity. In the site of Kuntillet ‘Ajrud, the petroglyphs show him seated along a female figure, and the accompanying paleo-Hebrew inscription reads “for Yahweh and his Asherah” (Hadley 1987: 51).

Asherah was a very important female divinity to the Canaanites, she was married to El and Baal and was the mother of their gods. El was the word meaning lord or god to the Cannanites, and was the head of the pantheon, Baal was a storm god who eventually replaced El in the myth The Ugaritic Baal Cycle (Smith 1994). So, to have him in a scene alongside her demonstrates that at the site of ‘Ajrud he would have held a similar role to El or Baal, ie; head of the Pantheon.

There are also the sites of Tel Dan, Tel Arad and Kirbet el-Qom which date to c. 9th/8th century BCE which depict Yahweh alongside other Canaanite gods, foremost the goddess Asherah. Additionally, Biblical evidence from 2 Kings tells us that in Jerusalem during this period they were worshiping other divinities such as Asherah and Baal.

I don’t know exactly what you mean by “how was the faith similar or dissimilar to its contemporaries”. I assume you mean the Canaanite faith with Yahweh in it? If so, it would have been very similar to contemporaries in the sense that it would have been hierarchically structured with Yahweh at the head of the respective pantheon. One thing to remember though, is that each community would have had their own version of the pantheon, so it's possible that whilst the community at 'Ajrud placed Yahweh at the head, others would have maintained worship in El or Baal and Yahweh would have continued to be a minor deity.

I hope this briefly answers your question.

For more info on this see:

Ackerman, S., (2013) ‘Goddess’ in S. Richard (ed.) Near Eastern Archaeology a Reader, Winona Lake, pp.391 -397.

Dever, W., (1984) “Asherah, Consort of Yahweh? New Evidence from Kuntillet ‘Ajrud”, Bulletin of the American Schools of Oriental Research 255, 21-37.

Dever, W., (2013) ‘Religion and Cult in the Levant: The Archaeological Data’ in S. Richard (ed.) Near Eastern Archaeology a Reader, Winona Lake, pp.383-390.

Hadley, J., (1987) ‘The Khirbet el-Qom Inscription’ Vetus Testamentum 37, 50-62.

Kaufmann, Y., (1951) ‘The Bible and Mythological Polytheism’ Journal of Biblical Literature 70, 179-197.

Smith, M., (1994) The Ugaritic Baal Cycle, New York.

Vriezen, K., (2001) “Archaeological Traces of Cult in Ancient Israel” in Becking, B., (ed.), Only one God? Monotheism in Israel and the Veneration of the Goddess Asherah, Wiltshire, pp. 45- 80.

Would this period even be called judaism? Is it Proto-Judaism?

Or just Judaism's antecedents?

What would be the terminology here? Is there a debate over what to call it?

What source says they had a polytheistic era?

This answer builds upon what asrafael answered, and delves deeper into the proto-Judaic religion.

While overall I concur about the idea of henothiestic idioms within pre-Babylonian Judaism (as is clear from the earliest Torahs discovered which date from 150BCE), I think analyse of the role of pre-history in the development of Judaism needs to take into account the unique idea of monotheistic religion. Up until the worship of the Arten in around the 1400-1300BCE in Egypt all recorded religion was predominantly poly or heno theistic. The idea of a singular 'I AM' was an aberration even to the Egyptian, with strong indications that the ruling dynasty who tried to impose the Aten were overthrown by the priesthood and polytheism restored.

Out of the milieu emerged the proto-Judaic religion, though any semblance of fact relating to what constituted actual Jewish beliefs prior to around 600BCE is complicated by the first Jewish diaspora and exile to Babylon. The Jews in Babylon came into contact with proto-Zoroastrianism, which bears striking similarities to a lot of beliefs that carried over into proto-Judaism once the Judeans were permitted to return to Jerusalem by Cyrus the Great in 538 BCE. How much of the pre-exile religion survived is debatable, as the miraculous discovery of the Torah in the temple by Hilkiah is extremely convenient in enabling the returning king to unify the people around the YHWH.

Prior to the diasphora the evidence points to a purely monotheistic religion that eschewed the trappings of angels, demons, good vs evil, heaven/hell etc. In the pre-exile Torah only 'I AM' exists separate and indivisible. At no point in the pre-Babylonian writings are any mystical or spiritual things mentioned (even the spirit of the Lord over the water is I AM), just that the singular God of the people who was a jealous god demanding sole worship, hence the first two commandments. When the people turned to the other gods they were severely punished. Baal, Asherah (consort of El), and various Egyptian deities all played a part in pre-diasphora life, only to be expunged once the Jews returned home and 'found' the Torah miraculously preserved in the Solomonic temple.

There is very strong evidence that first Zororastrianism, and then Platonic Greek philosophy under the Asasanid dynasty fundamentally changed the outlook of the pro-Judaic religion, which then developed the idea of a messianic figure to save the people from the invaders that conquered the nation. This notion is especially acute when you consider that the sadducees explicitly rejected the idea of a spirit world; they denied the resurrection of the dead, the existence of spirits, and the obligation of oral tradition, emphasizing acceptance of the written Law alone. It was only after the final destruction of the temple that the pharisetic traditions, combined with Greek philosphical influences, finally crystallised what we consider to be the modern idea of Judaism, monotheism, and the afterlife into a coherent post-temple faith.

There is also the other factor that proto-Judaism's traditions probably sprang from the faith of the Assyrian and Semitic tribes of the Tigris/Euphrates region, spreading to the semitic tribes in Canaan through trade and commerce, which is hinted at in the founding myth of Abraham (Ur being a major city in the Tigris/Euphrates region during the early civilising period). These traditions drew in, and mutated, the idea of El, Baal, and the other regional gods, though ultimately rejecting the notion of many in favour of an absolute one.

There is also the factor that post-exile Judaism was as concerned with salvation of the people in the form of the Massiah as it was about the law. The Pharasetic tradition, which became the rabbinic heart of what we consider Judaism, sprang from the need to worship without the temple, which is why the Jewish faith was able to survive the destruction of its holy site when many other polytheistic religions crumbled into history. This tradition emerged from the first diaspora, and enabled Judaism to move beyond its initial canaanite roots.

I appreciate that asrafael drew direct lineage from the migratory patterns of the proto-Semitic tribes, but personally I believe if you look at the writings and theological evidence available in terms of Judaism's embryonic roots you have to look north towards Persia, Edom, east towards Babylon and even India (Buddhism), and west to Greece for a fuller picture as to how Judaism's metamorphosis from one jealous god amongst many to a singular one without any other with all the trimmings evolved.

In summation, and to directly answer your question: Judaism's polythistic nature is an educated guess based around Torah scholarship and archaeology. Theologically what made proto-Judaism unique (outside of the cult of the Arten) was its emphasis on the I AM, as you term it YHWH. YHWH was the singular god of the people, an amalgamation of ideas and theologies that the canaanites, and in particular the Judeans, absorbed and crystallised into their patron deity at the exclusion of all others. Until the first exile Judaism was a religion of the Law (Torah), ritual, and sacrifice for atonement. After the exile it absorbed Greek, Zororastrian, and potentially Buddhist philosophies, but the overriding morphing of the pre-exile religion was the national desire for a Messiah to save the people. This Messianic complex was the real distinguishing feature, as while Monotheism by itself was revolutionary, without the Messianic impetus, coupled with the devolution of religious authority to the rabbis, proto-Judaism would never have evolved and migrated outside the Judean sphere.