In Sanhedrin 63  we have an argument between two Sages of the Mishna about what the problem with the Golden Calf was. They both agree it was  שיתוף [joining] something to God, but to the first sage that is not pure idolatry. To Rabbi Shimon Ben Yochai it is pure idolatry. Why not define idolatry as according to the number of gods one worships or the identity of the god? This is the unspoken problem I have been thinking about for a few days that got me to realize what the Talmud is getting at.  In the Talmud God is the Creator and everything else is created. To add anything to God and saying it has "godliness" is what the Talmud calls שיתוף joining.

To understand idolatry it seems you can classify it by  Advaita or pantheism will be on one end of the spectrum in the Far East. Then as you are on the longitude of Jerusalem you have Monotheism (God made the world and he is not the world). Then you go West and You have Monotheism, but with one person besides God also being God. And who you pick seems to be a matter of taste. The further you go West, the more you get materialism. In  the furthest west in Japan you get Buddhism which is zero-theism. And right between Japan and India the two lines meet so we find in fact Buddhism and Advaita being very similar. [Buddhism is atheism according to the Dalai Lama. I figure he must know.]

 Rav Shick [nicknamed "Mohorosh"] printed the books of Reb Nachman  Then started the period of the small pamphlets.

In his books, he would have regular statements of the Sages, but also throw in a statement of the Zohar איהו ממלא כל עלמין וסובב כל עלמין "He fill all worlds, and surrounds all worlds." And then he would add his signature statement אין שום מציאות בלעדיו כלל  "Nothing exists besides him." And sometimes also throw in his second signature statement  הכל אלקות גמור. ["Everything is pure godliness."]

In  letters he wrote "everything is the infinite light" הכל אור אין סוף ב''ה and "everything is the infinite one." הכל אין סוף ב''ה
All this comes from the Remak {Moshe Cordovero.} as quoted by the Shelah Hakadosh. That is probably where the Baal Shem Tov picked up the idea.

From what it is possible to tell this all might be an innocent mistake. We would say that nothing exists without God. Simple.  And this is all the Remak (Moshe Cordovero) meant. But it snowballed all out of proportion. By the time it got to the disciples of the Baal Shem Tov, Pantheism became the official doctrine. And Rav Shick being raised in a Satmar home thought it was traditional Torah thought. [And he never read the Guide for the Perplexed of the Rambam so there is no reason to think he would ever have become aware of authentic Jewish theology.]
He father was a friend and disciple of the Satmar Rav, Reb Joel.
Later on, this got mixed up with the Tzimtzum [contraction of the infinite light ] as a kind of way of defending pantheism.
With the Ari (Isaac Luria) we have none of this. Everything above Emanation is godliness, and everything below is not.  And that is straight from the Zohar itself. And this is in fact what we see in Nachmanides concerning the Golden Calf and the interface between God and his creation and his creatures. Obviously with the Rambam {Maimonides} only God will have godliness and everything else will not.

 In conclusion a taxonomy of idolatry will be how much outside of  God , the God of the Torah, the Five Books of Moses] is considered God. In the East everything.  That is Shankara. Then you move  a bit West and you get Ramanuja where there are gradations. Then on the longitude of Jerusalem you get Monotheism. Then in Europe you add one person. Until you get to Buddhism or zero-theism.

Now we understand the Talmud in Sanhedrin 63 about joining things to God being the problem with the golden calf. And we understand now why the Talmud takes this approach to idolatry and not the more natural thing to discuss the number of gods.
And now we can understand the Geon from Vilna (Vilnius). H could have put any number of groups into excommunication.  There were plenty of Shatz [Shabatai Tzvi] groups around. (Every city had its secret Shatz group especially in the Ukraine) But the Shatz was not claiming pantheism. Nor were his followers.
The Gra saw something more sinister in panentheism. He saw it as a sneaky way to direct worship towards people while pretending to be kosher.

Appendix and notes:

1) Reb Nachman said not to learn the Guide for the Perplexed of the Rambam nor any books that deal with Jewish Theology written by the Rishonim. And this is good advice from one aspect because those books are about orientation not learning Torah proper. Learning Torah proper means the Oral and written law, not books of theology. On the other hand for Rav Shick, the lack of knowledge about the מורה נבוכים the Guide had the result that he did not know that Judaism is Monotheism, not Pantheism. This was certainly in his case an honest mistake. He thought that when Reb nachman emphasized the importance of Faith that he was talking about pantheism when in fact Reb Nachman was referring to Monotheism. And this error has come to permeate all of Breslov including Na Nach.
[In Breslov looking at The Guide for the Perplexed or any book of authentic Jewish theology by any of the Rishonim (who were by all accounts the only people qualified to write such books) is considered a very great crime. I mean if you don't think the Rambam know what Torah is about then who does? But this creates the perfect storm. You have people intensely interested in what the Torah is about and yet can't open any authentic book of Jewish thought like Saadia Gaon or the Rambam to find out.  And after a few years these same people after spending all their time reading just Breslov books go out and write more Breslov books all in complete utter innocence of what the Torah says or means.
Now of course Reb Nachman himself is perfectly authentic and legitimate. He is simply coming from the school of thought of  Nachmanides and the Arizal. I have no complaints about that. On the contrary I find his books to be very helpful. It is just people that later write what they claim to be books based on Reb Nachman that I find to be problematic since they are always being written in ignorance of Torah

2) Brahman in Advaita is the only thing that exists. See the Bhagavad Gita with the commentary of Sankara for a detailed exposition of this point of view that is coming from a more religious perfective than Spinoza. Brahma the Creator of the Universe is created and exist for only one day of Brahman so this is not the same thing as  the Torah's point of view. (Spinoza is a bit close to Torah with his distinction between "Nature" and Nature naturing.")