I think men created God in their own image.
I think the idea of true monotheism, namely one absolute God that's almighty, start of in the beginning of almighty absolute monarchies. That is, during the age of empires.
True monotheism doesn't show up or be popular enough to become a state religion until Persia (zoroaster), China (Shang Ti), or Rome (Christianity).
Republics and democracy or less absolute monarchy tend to be polytheistic because their leaders need to be reminded that they too can be toppled. Absolute small monarchy like ancient judaism tend to be henotheistic. Only large empires start off true monotheism like what we have.
Rome become monotheists after they become monarchy. After that Byzantine's empire is split into different kingdom. Then their monks have a hard time explaining why the same God would tell 2 different christian kingdoms to kill each other. In ancient judaism, this is not an issue. Well, their God is YHWH and the other guys have different gods. Having many small kingdom and yet one God causes cognitive dissonance. Either religion or politics then change to match the other.
Religion is a very ancient way to impose our will against other. Rather than saying you got to do what I say, people tend to say, and still say, you got to do what my God says. So the kind of God I made up is decided by the kind of claim I found profitable to make.
First I need to differentiate between monotheism with polytheism because even in polytheist religions, one god is quite often supreme.
In both case, one God is often boss of all boss. El Elyon in Judaism and Zeus in ancient greek are both supreme. So simple hierarchy cannot be the difference.
Well, the difference between polytheist and monotheist is the relative power of their God. In polytheist religion, Zeus is boss over boss. However, if all the other gods unite against Zeus, he's death. Zeus has supreme political power but comparable muscle power with Poseidon, Hades, etc.
In monotheist, God is the boss and even if everything unite against him, He prevails.
In Judaism, God alone is not only politically powerful, but by himself almighty. That is the claim of most absolute monarchy. That the emperor is chosen by the almighty and need only to answer to him. That emperor will still be emperor no mater how many rebels are fighting him as long as the supreme God still favor him.
Greek before democracy are not as absolute monarchy as China. Seed of "equality of men" are in greek long through western europe, etc. Even Spartha is a republic and their king got to appease the people.
If I am very powerful, I would claim that my power is absolute. You just have to do what I said or else. So I will produce monotheism. Do what I said or else.
If I am a president, or a leader in republic or democratic countries, I would make a claim that I should be your leader because I am a good king. If I am not a good king you would kick my ass. So I too will produce a religion that reflect that. I would say, Zeus is boss, like I am boss. But well, if he pissed of Hades, Poseidon, or Hera he's screwed. So I am still your leader but I recognized that others have power to disagree.
So if the country is absolute monarchy, then God tend to be monotheistic. If a country is republic or democracy or recognize men capability to topple their king, the religion reflects that and tend to be polytheistic. The supreme god in democracy tend to get toppled a lot unless he get consensus from his followers.
Politic evolves faster than religion. So the link got weaken over time. However, original religions may correlate well with original government form.
In fact libertarian is effectively saying that I am free to do what I please and I do not want to obey you or your god, which I don't believe to exist anyway. So soon, atheism will be common too.
Religion is tale that enforce politics. Absolute monotheist/monotheist means a claim of absolute monarchy. Polytheists means a claim of non absolute monarchy. True monotheists means a claim of absolute monarchy for the whole world. Atheism means a claim for individual freedom.
So which one come first depends on which government system show up first. After a while, the link between form of government and number of God tend to get weaken.
Monarchies like israel tend to be monotheistic/henotheistic (Torah may be written during the monarch era). Republics like greeks tend to be polytheistic.
Ancient jews, persia, chinese are all monotheistic. One God, one King. One king per God.
The bigger the monarchy, the more "almighty" their view of God is.
In fact, true monotheism, where the God is almighty and is the only one out there, probably start of at the start of large absolute monarch empire like Persia, Roman, or Chinese. Before it's either polytheist (in case of republics) or henotheist (for small country like israel).
There is one God. That one God appoint one Emperor. Me. So all of you got to do what I say. That's the message of monotheism.
Imagine if I am an emperor and says, there are many Gods. Zeus better behave lest Hades kicked his ass. Then the people would say, you too should behave o emperor, lest we kicked your ass.
Religion is a way to impose your will against others.
So the nature of god that a person made up tend to correspond to the nature of power that he possess.
If I am just a leader of a small kingdom, it's not toward my best interest to say, my God is almighty all of you have to bow before Him and pay me tribute and give your daughter as my sex slaves. Saying that would provoke a bigger country to just smash me and said, "where's your God?" Happened to jews a lot.
This is why ancient big countries like chinese/persia believe in an almighty single God where smaller country like israel believe in a henotheistic God.
In a sense, ancient jews didn't claim that their God is almighty. Christians claimed that given that Christian is the official religion of the mighty roman empire.
In China, Shang Ti is the God/emperor of all and chinese emperor is emperor of everything under heaven. So all this time, according to my ancestors, all of you are chinese. You just don't know it. It's not until the Qing dinasty that the chinese start seeing that they are just a nation among nation.
There are other gods/spirits. They all work for Shang Ti. He's a boss of boss sort of. Emperor above. God most high. Shang Ti (supposedly) pick only one emperor and that one emperor is emperor of everything under heaven. That's what chinese used to believe. How can chinese believed that? Because chinese emperor said so. How can chinese emperor make that claim? That's because he's an emperor of a very big country.
Persia as a large empire have that same point of view with zoroaster.
Judaism is strange. If you ask a jew, I bet they said that from the beginning they have been worshiping an omnipotent, religious tolerant, democratic God that opposes stoning.
It seems though that many part of jewish's God is borrowed from countries they've been around with.
Original judaism seems to be henotheistic. There shall be no other God before me doesn't mean there is no other gods. It could mean that even when there are other gods you're not supposed to worship that one.
A sample is deuteronomy 32:8. Most ancient scrolls jot that down differently suggesting that many different bible compiler may think that the original is too controversial they ended up changing it.
for example, noted how The Most High, divide (the 70 nations) according to the number of children of God. Ancient Canaanites pantheon also had 70 children. YHWH portion is Israel. So it seems that the original view is that Yahweh is the God of Israel and Israel alone. The other 69 nations have their own God that is comparable to YHWH.
I was surprised when I found that finding part of the tanach that support unambiguous monotheism before babylonian exiles is not easy. Do you know any?
Then jews become foreigners in many places. From Persia they adopt monotheism and religious tolerant. From western europe they adopt democracy and equality. The almighty aspect of God seems to come from Plato, which then become the almighty God that we're familiar with.
Again I may be wrong. I hope there are other historical records that show otherwise. If so, please tell me.
For example, is there any jewish comentary that claim that YHWH is almighty before Plato, for example. That'll be interesting. It seems that the idea that YHWH is almighty comes from Plato rather than from original judaism.
Being able to split seas doesn't mean you're almighty. In fact, we humans can do that easily now.
Curiously, the word "El" in Judaism can be used for gods or powerful being to further add confusion. Curiously too, jewish bible translation like http://www.mechon-mamre.org translate El as God anyway even when modern Judaism claim that it's not God. Such as when Jacob wrestle with El, mechon-mamre would translate Israel as struggle with God and Peniel as face to face with God even though modern judaism believe that Jacob is wrestling with an angel rather than God.
In all cases, judaism believe in many "El"s.
Now what about republic or democracy? Well, the greek is democratic. The Canaanites are republic or federal. So they tend to have multiple gods.
Zeus, Cronos, may just be a model of ancient king that got toppled for being a tyrant. I think that's the true message of polytheism. Leader better behave lest you got toppled.
Note: When I said God here, I mean images of God that people actually believe and worship. How the actual true God behaves is out of my capability to see and I really want to know more. It is possible, as 3 billions (and growing number of) people believe, that YHWH is indeed "The God" equal to (the same guy as) Shang Ti. However, historical evidence seems to show that original jews didn't believe that their God is almighty or the only one around. Mighty, but not almighty.