Lawrence H. Schiffman, Texts and Traditions, Ktav, Hoboken 1998, p.735.

The Mishnah outlaws the public teaching of esoteric religious traditions regarding creation and the divine chariot or throne. The traditions alluded to were part of a corpus of teachings tracing their origins to Second Temple period sources and continuing to develop alongside and in relation to Rabbinic aggadot on the same subjects.

The forbidden sexual relations may not be expounded before three [or more] people, nor the account of creation 208 before two, nor the Chariot (Ma‘aseh ha-Merkavah) 209 before one, unless he is a Sage who understands by his own knowledge. Whoever speculates about four things, it would have been better for him if he had not come into this world- what is above, what is below, what was before, and what will be afterwards. Whoever is not sensitive to the honor of his Creator, it would have been better for him if he had not come into this world.

207. Trans. S. Berrin.

208. Genesis 1.

209. Ezekiel 1.