Kabbalah
Kabbalah means "that which is received". It is the Jewish mystical tradition, and it is older, stranger and more textual than the version most mystery schools teach.
It has layers. The earliest is the chariot mysticism of the first centuries CE, meditations on Ezekiel's vision of the throne of God, recorded in the Hekhalot literature. From the same period, somewhere between the second and sixth centuries, comes the Sefer Yetzirah, the Book of Formation, a text short enough to read in an hour. It says that God created the world through thirty-two paths of wisdom: ten sefirot, which are numbers before they are anything else, and the twenty-two letters of the Hebrew alphabet, divided into three mothers, seven doubles and twelve simples. To each letter it assigns a piece of creation: an element, a planet, a sign of the zodiac, a part of the body, a faculty of the soul. Creation, in this book, is speech. Everything on the Tree of Life descends from those few pages.
Kabbalah proper begins in the twelfth century with the Bahir in Provence, and reaches its height in Castile in the 1280s with the Zohar, a vast mystical commentary on the Torah written mostly by Moses de León and attributed to a second-century rabbi. The Zohar turned the sefirot from numbers into a living structure of divine emanation, the Tree as it is now drawn. In the same generation Abraham Abulafia taught a practice of letter permutation built directly on the Sefer Yetzirah. Three centuries later, in Safed, Isaac Luria added the drama of contraction, shattering and repair that gives Kabbalah its account of exile and redemption, and from Luria the tradition passed into Hasidism, where it still lives.
The West met Kabbalah through Christians. Pico della Mirandola read it in Hebrew with a Jewish tutor and in 1486 declared that it proved the divinity of Christ. Reuchlin followed in 1494 and 1517, Agrippa gathered it into his occult philosophy in 1531, and in 1677 Knorr von Rosenroth published the Kabbala Denudata, a Latin digest of the Zohar that became the main source for everyone who could not read Hebrew, which was almost everyone. Christian Kabbalah is a real tradition with real scholarship in it. It is also Kabbalah at one remove, read for what it could be made to say.
The mystery schools took a further step. The Golden Dawn claimed the Sefer Yetzirah as its authority, and drew the Tree from a diagram in a Jesuit's book of 1652. But when it came to the seven double letters and the planets the Sefer Yetzirah assigns to them, the founders set the text aside and used an arrangement of their own, and that arrangement, with Lévi's Tarot laid over it, is what nearly every book on the Tree of Life has taught since 1888. Lucian Kabbalah, which is the reason this site exists, is an attempt to return to what the Sefer Yetzirah actually says, using Peter Hayman's critical edition of the manuscripts as the textual authority.
You can recognise this root by its furniture: the Tree of Life, the ten spheres and twenty-two paths, Hebrew letters on Tarot cards, the divine names, the seventy-two angels. If your school has any of these it is drawing on Kabbalah. The useful question is how far down the chain it is drawing from: the Hebrew text, a Latin digest of it, or a nineteenth-century order's reconstruction. Each is a different distance from the source, and the distance shows.
Gave rise to: Western Esotericism, Solomonic grimoires, Christian Kabbalah (Reuchlin 1494, Agrippa 1531), Golden Dawn: the second confluence, Lucian Kabbalah (Hayman 2004 as textual authority).