Bhagavad Gita · Chapter 18: Freedom through Renunciation

Bhagavad Gita 18.42

śamo damastapaḥ śaucaṃ kṣāntirārjavameva ca | jñānaṃ vijñānamāstikyaṃ brahmakarma svabhāvajam ||

Calm, self-control, austerity, purity, patience, honesty, knowledge, realization, and faith: these are the duties of a brahmin, born of his nature.

Modern context

The qualities of the brahmin's nature: calm, self-control, discipline, purity, patience, honesty, knowledge, realization and faith. Notice that this is a list of qualities, not privileges. By this description, a person is a brahmin by character. Someone born into a priestly family without these qualities does not fit the description; someone with them, whatever their birth, does.