I have briefly developed here some of the fundamental concepts of the Book Yesirah because they are of essential importance for the understanding of what follows and because this book was later read and interpreted by the kabbalists as a vade mecum for the Kabbalah. In contrast to later interpretations, the special charm of this text consists in the frequently felicitous and in any event ever-vivid imagery and fullness of meaning it lends to most of the concepts newly created in order to express abstractions. The author finds concrete and appropriate designations for notions that, until then, Hebrew did not know how to render in adequate terms.
That he failed on certain points and that his images sometimes remain obscure for us—which only encouraged their subsequent reinterpretation—is a clear sign of the difficulty of his efforts and of the energy with which he undertook them. The book’s solemn and enigmatic manner of speaking made it possible for the Jewish philosophers as well as the kabbalists of the Middle Ages to appeal to its authority.
Saadya, in the earliest extant (although certainly not the oldest) commentary interpreted it around 933 in accordance with his philosophic conception of the doctrine of Creation and his Jewish theology in general. Since then, a complete series of more or less detailed Hebrew and Arabic commentaries continued to be written down to the thirteenth and fourteenth centuries.
Everyone found in the book more or less what he was looking for, and the fact that Yehudah Halevi devoted extensive attention to it, almost a complete commentary, in the fourth tractate of his principal work of philosophy and theology, Sefer ha-Kuzari (around 1130), may serve as an indication of the great authority the book enjoyed.
But at the same time, this text also remained influential in entirely different circles, those who saw in its theory of language some sort of a foundation of magic, or those for whom the doctrine of the book included authentic elements of the Merkabah gnosis and of cosmogony.
The Book Yesirah was studied in the schools of the sages of Narbonne as well as among the French rabbis of the school of the tosafists and among the German Hasidim of the same period, and many commentaries have come down to us from these circles, which were generally averse to philosophic speculation.
It offers remarkable parallels, to say the least, to the turn which the Kabbalists gave to the doctrine of the sefiroth. It is no longer possible to say with certainty to what extent the study of the Book Yesirah was regarded in these circles as an esoteric discipline in the strict sense of the term. Perhaps one could view the text as situated at the limits of esotericism, partly within it, but partly already beyond it.
–Gershom Scholem, Origins of the Kabbalah, 1987, pp. 33-5.