The Book Bahir, (anonymous, 1185), attributed to Rabbi Nehunia ben ha-Kanah, “begins with a few statements concerning the creation. In the first part of the book there are many discussions of the letters of the alphabet, their shapes, and the meaning of their names.”
“This work is the first Jewish treatise that presents in a positive manner the concept of transmigration of souls, the reincarnation or rebirth of the same souls again and again.”
(I had no idea that reincarnation had any place in Jewish Kabbalah).
This work is technically the earliest work of the Kabbalah, based on three major concepts which are not found in earlier Jewish sources.
The first is the description of the divine world consisting of ten hypostases, ten divine powers, which are called ma’amarot (utterances), which were known in later kabbalalistic writings as the ten sefirot.
The second is the identification of one of the ten divine powers as feminine, separate from the other nine, and thus introducing gender dualism into the image of the divine realms.
The third is the description of the divine world as a tree (ilan); the work states that the divine powers are positioned one above the other like the branches of a tree. But the image was one of an upside-down tree, its roots above and its branches growing downward, toward the earth.
These three concepts became characteristic of Kabbalah as a whole, (excepting Abraham Abulafia, who rejected the concept of the ten sefirot), and the presence of these three concepts identifies works as part of the tradition of Kabbalah.
“In addition to these three concepts there is in the Book Bahir a more dramatic description of the realm of evil than those usually found in earlier Jewish sources, but there is no final separation between God and Satan. The powers of evil are described as the fingers of God’s left hand.”
–Joseph Dan, Kabbalah: A Very Short Introduction, 2006, pp. 20-2.