Study of esoteric teachings was not merely theoretical.
It seems to have been accompanied by profound mystical experiences which were apparently dangerous for those imperfectly prepared.
These experiences were known, we believe, as pardess (entering the orchard), and there is a well-known story in the Talmud about four sages who entered this "orchard":
R. Akiva, Simeon Ben Zoma, Simeon Ben Azai, and Elisha Ben Abuya.
They were guided by the wisest and most experienced among them, R. Akiva, and it is related that he warned them of certain dangers awaiting them in words that could have no meaning for those who had not travelled in those spheres.
Despite his guidance, however, the group was unable to withstand the dangers:
Ben Azai died, Ben Zoma lost his reason, and Ben Abuya "uprooted plants," that is, arrived at heretical views, apparently under Gnostic influence.
Only R. Akiva "went in in peace and came out in peace."
This story is the most detailed but not the sole description extant, and its intention was to emphasize the dangers awaiting those who entered into this domain.
The teaching of Ma'aseh Bereshit (concerning creation) was therefore never carried out in public, and was always confined to one disciple.
Thus a student whose qualifications were closely examined did not receive detailed instruction but was merely taught outlines of the subject.
In this way, if he did not display aptitude and did not himself arrive at a mystical experience, he would not be harmed by the knowledge imparted to him.
We find, for example, that R. Elazar Ben Arakh lectured on this subject to his teacher, Rabban Yohanan Ben Zakkai, who was amazed at the understanding he displayed.
Other disciples told Yohanan of their discoveries in this sphere, and he replied:
"These words were said to Moses on Sinai."
–Rabbi Adin Steinsaltz
From The Essential Talmud by Rabbi Adin Steinsaltz