You can get some idea of the origins of Hasidism as a mystical Jewish movement by reading about the Rabbi Yisrael Baal Shem Tov. This writeup focuses on the roles of acosmism and concepts of holiness in the development of Hadisism.
Early in the history of Hasidism, notable rebbes attempted to define the movement. Two central ideas arose. One was that Hasidism was a way of “working on oneself”—a path of self-improvement. Another, put into words by Dov Baer, in a private letter, was that “all is God.” The first has guided Hasidism a great deal, and is visible today in the strict Orthodox religious observance and moral teachings of modern Hasidic Judaism. The second, termed acosmism (and quite similar to pantheism), was a revolutionary statement in its time, and still does not reconcile easily with the dualistic traditions of Jewish theology. Nevertheless, it has exerted subtle but important influence on the developments of both Hasidic and Renewal Judaism.
One of the problems that Dov Baer’s statement poses is its seeming contradiction of the concept of kadosh (often translated as “holiness”), an idea that goes back to ancient Israelite religion. Kadosh joins together the notions of the separateness and the holiness of God, and is thus the source of a dualism between Heaven and Earth: that which is of the world must be separated from the sanctity of the divine. This is hard to make sense of if all is God. Kadosh was and is important to Hasidim, however, and Hasidism continued to stress observance of mitzvot (commanded good deeds) and halacha (Jewish law). The most radical rabbis conceded that theoretically halacha was unnecessary, because all was holy, but argued that it would not be right to live without traditional law until the time of the messiah.
Another example of an idea that potentially conflicts with acosmism is devekut, meditation and prayer with the intent of “cleaving to God.” Devekut was also a strong aspect of early Hasidism, but its very name suggests a separation between God and the self. This contradiction may be fine, though, if one distinguishes between reality and his understanding of it: all is in fact God, but to realize this one must come closer to his own image of God through devekut. In addition, from a historical perspective devekut makes sense within Hasidism because it was not something new, but was instead a common feature of kabbalistic tradition. Hasidic thought in fact takes devekut a step further by connecting it to acosmism: if God is all, then, as Sefat Emet wrote, “every person can attach himself [to God] wherever he is, through the holiness that exists within every single thing, even corporeal things.”
This attachment to God takes work, however. In this way, devekut is related to the other definition of Hasidism as well: it is necessary to “work on oneself” to reach even the lowest rungs of devekut, and the higher rungs require even more spiritual self-improvement. This necessity of working on oneself to achieve devekut makes clear another aspect of Hasidism: that, even if all is God, one is not automatically in constant communion with the divine.
In addition, achieving devekut through the world around us requires consciousness that God is within the world. Many would argue that this consciousness is in fact necessary to live a fulfilling life: Isaiah of Dinovitz made a distinction between taking pleasure from the world as we perceive it and from the true source of the beauty in our world. “Fascination with the world as it appears separate from God,” Miles Krassen writes, “is tantamount to idol worship.” Dualism, then, continues to be an important idea in Hasidism, through the emphasis placed on honoring the Shekinah (presence of God) within the world around us instead of honoring the world itself. This distinction is based on the idea that God’s presence is everywhere, but still distinct from the material world it inhabits. This makes Dov Baer’s acosmism less than literally true, but at the same time demonstrates the influence of its omnipresent element.
Indeed, acosmism has never been a truly pervasive feature of Hasidism: not all Hasidic rebbes and thinkers agree that all is God. More than a concept that is agreed upon by all, then, it is a concept that has influenced all: even modern, devoutly Orthodox Hasidim see God as immanent in their lives. In the history of the Hasidic movement, many teachings have been based upon the concept. The Keter Shem Tov quotes a parable told by the Baal Shem Tov, in which he describes a king creating illusory walls which seem to separate his subjects from him. Viewed through the lens of Hasidic acosmism, this can be interpreted quite clearly: the father is God, and the illusions are our perceptions of the world around us. God is here despite our difficulty in seeing him, but the material appearance of the world makes the realization of this truth a challenge.
The idea of holy sparks written about by Isaac Luria and taught by the Baal Shem Tov also hinted at acosmism without requiring that all be made of God. It states that there are sparks of divinity everywhere, and that one can gain strength from objects that contain these sparks, such as food or clothing, and use the strength to serve God, elevating the sparks themselves to purer holiness. In this view, the presence of God is truly spread everywhere, but in a reduced form, and only through human deeds can the presence be used for good. In a related teaching, the Baal Shem Tov explained that the Shekinah is actually made up of the Jewish people.
The Piesetzner Rebbe, on the other hand, claimed a deep spiritual significance in the literal claim that all is God. He made an explicit connection between acosmism and humility, writing that “the Holy One fills all creation, being is made of God, you and I, everything is made of God… while I, in my stubborn insistence on my own autonomy and independence, only succeed in banishing myself from any possibility of meaning whatsoever.”
The Maggid of Mezritch shared some of these conceptions of humility. His use of the term ayin, or nothingness, to describe the ideal self-image reached through devekut gives importance to the contrast between God and the self. The idea of not being during prayer, of transcending the ego by not even thinking of self-existence, is one frequently described in Dov Baer’s writings, such as Maggid devarav le-Ya’aqov. It is also related to his acosmism because belief that all is God makes transcendence of the ego easier: one need not work to act as though insignificant before God if he believes that he is really just a small piece of this infinite God.
The concepts of ayin and its opposite, yesh, allow the non-dualistic concept of acosmism to be expressed in dualistic terms familiar to Judaism. In using these words, one accepts that there is nothingness, ayin, and being, yesh, but one need not interpret the former as God and the latter as the world. Instead, “the deity,” writes Rachel Elior, “is presented as possessed of two opposite but interrelated aspects.” The concept of the world we see being an illusion, expressed by the Baal Shem Tov, is relevant again here, as the yesh is what one sees, but the ayin is always behind it. Because we see and appreciate the yesh everyday, we fully understand it, so it is only the hidden ayin aspect of God that we must work to understand.
Like older Hasidic traditions, Renewal Judaism reveals the influence of acosmism on inspection, mostly in its views on the immanence of the Shekinah and the universal truth of religious traditions. The former set of ideas are quite similar to those of other forms of Hasidism, and are expressed similarly by Zalman Schachter-Shalomi, founder of the Renewal movement: “we see how Devekut is a form of practicing the Presence of G~d, seeing G~d’s immanence in all Creation.” The latter is rather original, however. Renewal Judaism’s acceptance and appreciation of other religious traditions can be interpreted as stemming from the view that God is all, or at least that God is everywhere and accessible to all, and thus any people, not just the Jews, can create a valid and meaningful religion. “As plants are helio-tropic beings,” wrote Schachter-Shalomi, “growing toward the sun, so are we humans (and all of creation) Theo-tropic beings, growing toward G~d. There are thus basic similarities in all methods of spiritual direction.”
This revelation relates back in time to the Hasidic midrash of seeing the aleph (א), included in Zera Kodesh by Naftali Tsvi Horowitz of Ropshitz. This text describes how the letter aleph is both a form of the name of God and the appearance of a human face (with the diagonal forming the nose and the two smaller lines forming the eyes), and that God can thus be seen in the face of every human. Like acosmism, this concept communicates the universality of God in a radical way: God’s presence is not limited to Jews or to Hasidim, but surrounds everyone. “In holiness, we are radiant,” wrote Horowitz.
This idea gets at an important interpretation of acosmism: being a part of God does not necessarily dehumanize a person, but can in fact make that person more spiritually fulfilled, and hence, from the Hasidic point of view, more human. In part because acosmism does not deteriorate the personal mysticism of Hasidism, it has survived as an influence on the tradition through its entire history.