In Parts I and II (see
https://jack.dreamwidth.org/tag/boyarin-apocalypse), we saw Elisha Ben Abuya aka "Aher, the Other rabbi" get shunned for believing something heretical to ardent monotheists. The story describes four great rabbis entering a metaphorical "garden" and him seeing seeing Metatron giving orders from a golden throne, and crying "God?", and then legions of angels drag Metatron away to be whipped him for temerity. But why are they punishing Metatron here?
This was some sort of dualistic or binitarian belief, but what? This was central to the point Daniel Boyarin was making in these lectures. He's showing that despite quite a lot of strands of Jewish mysticism having no written records from about 300BCE to 200CE, they didn't vanish during this time but were probably actually common but disapproved of. This is an extended swipe at another unparalleled biblical scholar, Peter Schafer, who wrote books about ways Christianity did influence Judaism.
At some point Schafer described one of Boyarin's books using the "what is good is not new and what is new is not good" quip. Another time, he says "some scholars" believe Christianity didn't influence Judaism at all, but gives an example of a book by Boyarin. Boyarin angrily retorts Shafer his putting words in his mouth, he's saying THIS PARTICULAR story with the garden was an influence of a different tradition, not Christianity, and gave this fascinating lecture series to explain why in extreme detail.
Boyarin takes a number of specific examples that appear in the Talmud. For instance, the garden story was supposed to be a warning "IF YOU KNOW HOW TO GO TO HEAVEN DON'T WRITE IT DOWN THE KNOWLEDGE IS TOO MUCH FOR MOST PEOPLE" but the rabbis of a few hundred years later helpfully go into a lot of specific detail, including specific steps Rabbi Akiva supposedly undertook and the substances of seven onion-layers of heaven he travelled through.
Boyarin compares these to similar stories recorded earlier (e.g. 1 Enoch where Enoch travels to heaven in a similar way and is taken up as an angel taught great mysteries about everything) and later (e.g. 3 Enoch where someone else travels to heaven and meets Enoch-now-Metatron who complains about getting whipped for it, but gives him important visions), and demonstrates that these probably all influenced each other, as opposed to the ideas being re-imported from Christianity or another source. Conversely there's a story in Daniel about a great king and Someone sitting on a great throne and Daniel getting important visions, in both Jewish and Christian tradition, but this seems to be a slightly different version of the story emphasising who is in heaven and how they rule, not a human becoming an angel and getting whipped for going too far.
These likely relate to a much earlier story. The Sumerian King List records a sequence of kings stretching back before a Great Flood, and the seventh antediluvian king has a series of legends about being taken up into heaven, being allowed to run humanity and making a golden age, and then screwing it all up by being worshipped too much for his own sake and cast down again. Legends about a junior divine being who had been, or became, human, or who got too big for his boots and had to be cast down float about since then. The Sumerian religion at the time was later reformed by Zoroaster as Zoroastrinism.
This seems to be the earlier point of the Enoch-Metatron stories. The bible only says Enoch (seventh generation patriach) went off with God instead of dying. But this is back when the idea was that heaven was for angels and dead people went to some sort of underworld, so it may reflect the "went to heaven and was taught all the mysteries and became an angel" story recorded later. Then some Jewish people happily bought into the "Metatron rules on some days when God is busy doing accounts" plan whether or not Metatron had been Enoch. Others told them off with the "Yes, and he was PUNISHED FOR IT" bit of the Enoch/Metatron story. This is why Metatron's punishment for what Aher said seems to come out of nowhere.
Conversely, if Jesus or the Holy Spirit has any relationship to this story, they don't have any of those bits, so the bit with Metatron being seen by the Rabbis going into the garden derives from earlier Jewish tradition, not a denouncement of contemporary Christianity. And so (says Boyarin), Boyarin's long detailed lecture series is correct and Shafer's throw-away snarky footnote is wrong.