The most ancient declaration of Esoteric Teaching in the Sacred Books of India is probably this verse from the earliest and oldest of the ten collections of Rig Vedic hymns:

Chatvâri Vâk parimitâ padâni
Tâni vidur Brâhmanâh ye manîshinah
Guhâ trîni nihitâ na ingayanti
Turîyam vâcho manushyâh vadanti.

Dr. John Muir, most careful and exact of Orientalists, translates this verse:

Speech consists of four defined grades.
These are known by those Brâhmans who are wise.
They do not reveal the three which are esoteric.
Men speak the fourth grade of speech.

In this ancient hymn, the word Brahman means a Knower of Brahma, a Knower of the Eternal. It was only in far later times that Brahman came to mean a member of a hereditary caste.