The mysteries of creation — the Maaseh Bereshit — were considered so dangerous that the sages restricted who could study them. The Talmud (Hagigah 14b) famously records the story of four sages who entered the Pardes, the "orchard" of mystical knowledge. Only one, Rabbi Akiba, emerged unscathed.
But before that famous incident, earlier sages had already demonstrated the power and peril of expounding creation's secrets. The Talmud records that when certain scholars began to teach publicly about the workings of the divine chariot — the Merkavah (the Divine Chariot) described in Ezekiel — fire descended from heaven. Not as punishment, but as confirmation: the teaching was so powerful that the spiritual world responded visibly.
The rule was established: the mysteries of creation may not be taught to more than one student at a time, and even then, only to a student who is wise and understands on his own. You cannot mass-produce mystical knowledge. Each student must be guided individually, because the same teaching that illuminates one mind can shatter another.
Rabbi Yohanan ben Zakkai once rode a donkey while his student Rabbi Elazar ben Arakh walked alongside and asked to hear some of the chariot mysteries. Rabbi Yohanan agreed. Rabbi Elazar began to expound, and immediately fire surrounded them and angels gathered to listen, as at Mount Sinai.
"If even the angels desire to hear these teachings," the sages concluded, "how carefully must we guard them from those who are not ready?"