Where Did Monotheism Come From?

Judaism introduced two revolutionary ideas to the ancient world. The first was monotheism: there is only one God, Who created the universe and Who expects some kind of level of moral behavior from the people He created. The second was abolition of human sacrifice. (I have read that the Hindus abolished human sacrifice about the same time as the Jews; but casual googling gives me the impression that human sacrifice has died out gradually among the Hindus, starting around 500 BCE, and has persisted sporadically into modern times.)

So where did monotheism come from?

The following sentence is phrased to try to convince even people who don't believe in an Abrahamic religion. The traditional narrative of the ancient Israelites, as recorded in Genesis, is that their ancestors came from Mesopotamia in the early second century BCE and that those ancestors worshiped a single God.

I also assume that David Ulansey is headed in the right direction about Mithraism. You can see his take on Mithraism here and here. Briefly, Mithraism was a (sort of) monotheistic mystery cult that was popular in the Roman Empire until it was suppressed by the Christians. The earliest date claimed for evidence of the existence of the cult is 67 BCE. The Mithraic god was Mithra, the god of heaven.

The central image of Mithraism was the "tauroctony", an image of a man killing a bull.

To understand Ulansey's take on Mithraism, you need to know what the precession of the equinoxes is. The first two Google hits on "precession of the equinoxes" are to the Brittanica.com article and the Wikipedia article. Start with the Brittanica.com article first because it is much more concise than the Wikipedia article. Then skim the Wikipedia article for whatever interests you there. If you are having trouble visualizing what is going on, try this video. (You can also try YouTube but beware of videos with agendas.)  Briefly, the Earth's axis wobbles, with a period of about 26,000 years. As a result, the zodiacal constellation in which the Sun rises on the first day of Spring keeps changing. For about the past 2000 years, the Sun has been rising in Pisces on the first day of Spring, and we have been in what the astrologers call the Age of Pisces. Between about 2200 BCE and the year zero, during the Age of Aries, the Sun rose in Aries on the first day of Spring. Between about 4400 BCE and 2200 BCE, during the Age of Taurus, the Sun rose in Taurus on the first day of Spring.

Ulansey's thesis is that the tauroctony shows Mithra the god of heaven killing Taurus to usher in the Age of Aries.

The precession of the equinoxes was discovered by the Greek astronomer Hipparchus in abut 125 BCE. Ulansey proposes that sometime in the following 50 years or so someone, presumably somewhere in the Mediterranean basin, invented a new mystery cult based on Hipparchus' discovery, with the name of the (only?) (main?) god of the cult being borrowed from the Zoroastrian pantheon to make the cult sound exotic.

My problem with that foundational hypothesis is: Why a bull? Why not a ram? In about 100 years the vernal equinox (the point in the zodiac where the sun is on the first day of Spring) is going to move from Aries to Pisces. Wouldn't it be more to the point to depict the Master of Heaven as ushering in the Age of Pisces?

I propose that what eventually became Mithraism started more than 2000 years before, in The Theological Crisis of the 24th Century BCE. I conjecture that during the Age of Taurus Mesopotamian priest-astronomers developed an elaborate (poly)theology, one of whose essential components was that the vernal equinox has to be in Taurus. In the 24th century BCE these priest-astronomers saw to their horror that the vernal equinox was continuing to move west through Taurus and in about 100 years or so would move into Aries. To some of the priest-astronomers, this could only mean that the gods they had been worshiping were mere servants of a Master God Who dwells outside the observable universe and Who directs the affairs of the universe through His servants. Of course, these heretics had to keep their heresy secret.

This conjecture also addresses some of the other weaknesses in Ulansey's theory that have been identified by his critics. For example, Shepherd Simpson wrote:

Prof Ulansey argues that the characters are constellations and that the constellations shown are those which lay on the Celestial Equatorduring the period between about 2000 BC and 4000 BC - his dates for the Age of Taurus. This, he argues, is how the tauroctony is able to depict a particular moment in time: owing to the Precession of the Equinoxes, the constellations which lie along the Celestial Equator change slowly with time, so a particular constellation list indicates a particular time period.

...

Finally, the other question which arises is - even for a skilled astrologer - would it have been possible at the time to calculate which constellations would have been on the Celestial Equator during the Age of Taurus? To say the least this would have been an impressive feat! We have no indication that this was possible, nor any indication that any astrologer of the time ever attempted or achieved such a task.

 The heretical priest-astronomers didn't need to calculate anything. They just had to look up at the sky to see which constellations were on the celestial equator.

Several centuries later, some of the heretics migrated (fled?) west to the shores of the Mediterranean. Some of the migrants were the ancestors of the Israelites. The other migrants kept their heresy entirely secret until the precession of the equinoxes was rediscovered and published by the unbelievers and it was no longer necessary to preserve total secrecy.