Bhagavad Gita, Chapter 18, Verse 42

The control of the mind and the senses, austerity, purity, forbearance, and also uprightness, knowledge, realisation, belief in a hereafter, these are the duties of the Brâhmanas, born of their own nature.

A real, specific list of inner qualities is given here to define this first role, not just a job description.

Tranquility and integrity are named alongside knowledge and wisdom, showing this role is defined by real character, not by external duties alone. Substance, not title, is what actually qualifies someone.