Origins of Jewish Esotericism.
The origins of Jewish esotericism derive from a talmudic statement made in the Mishnah (Hagiga 2:1), circa 1st century CE. It was forbidden to expound two particular sections of scripture in public, and hazardous to even study them in small groups. The sections are the chapters of the Book of Genesis, describing the creation of the cosmos, called ma’aseh bereshit (the work of genesis) in the Talmud.
The second section was the first book of the Book of Ezekiel, called the ma’aseh merkavah (the work of the chariot), being the prophet Ezekiel’s description of the vision of the celestial chariot in Ezekiel 1 and 10.
They were regarded as spiritually and even physically dangerous.
—-Joseph Dan, Kabbalah: A Very Short Introduction, 2006, pg. 11.