mukhato ’vartata brahma
yas tunmukhatvad varnanam
mukhyo ’bhud brahmano guruh
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
O chief of the Kuru dynasty, the Vedic wisdom became manifested from the mouth of the virat, the gigantic form. Those who are inclined to this Vedic knowledge are called brahmanas, and they are the natural teachers and spiritual masters of all the orders of society.
Purpor by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupadat:
As confirmed in Bhagavad-gita (4.13), the four orders of human society developed with the order of the body of the gigantic form. The bodily divisions are the mouth, arms, waist and legs. Those who are situated on the mouth are called brahmanas, those who are situated on the arms are called ksatriyas, those who are situated on the waist are called vaisyas, and those who are situated on the legs are called sudras. Everyone is situated in the body of the Supreme in His gigantic visva-rupa form. In terms of the four orders, therefore, no caste is to be considered degraded because of being situated on a particular part of the body. In our own bodies we do not show any actual difference in our treatment towards the hands or legs. Each and every part of the body is important, although the mouth is the most important of the bodily parts. If other parts are cut off from the body, a man can continue his life, but if the mouth is cut off, one cannot live. Therefore, this most important part of the body of the Lord is called the sitting place of the brahmanas, who are inclined to the Vedic wisdom. One who is not inclined to the Vedic wisdom but to mundane affairs cannot be called a brahmana, even if he is born of a brahmana family or father. To have a brahmana father does not qualify one as a brahmana. The main qualification of a brahmana is to be inclined to the Vedic wisdom. The Vedas are situated on the mouth of the Lord, and therefore anyone who is inclined to the Vedic wisdom is certainly situated on the mouth of the Lord, and he is a brahmana. This inclination towards Vedic wisdom is also not restricted to any particular caste or community. Anyone from any family and from any part of the world may become inclined to the Vedic wisdom, and that will qualify him as a real brahmana.
A real brahmana is the natural teacher or spiritual master. Unless one has Vedic knowledge, one cannot become a spiritual master. The perfect knowledge of the Vedas is to know the Lord, the Personality of Godhead, and that is the end of Vedic knowledge, or Vedanta. One who is situated in the impersonal Brahman and has no information of the Supreme Personality of Godhead may become a brahmana, but he cannot become a spiritual master. It is said in the Padma Purana:
avaisnavo gurur na syad
vaisnavah sva-paco guruh
An impersonalist can become a qualified brahmana, but he cannot become a spiritual master unless and until he is promoted to the stage of a Vaisnava, or a devotee of the Personality of Godhead. Lord Caitanya, the great authority of Vedic wisdom in the modern age, stated:
kiba vipra, kiba nyasi, sudra kene naya
yei krsna-tattva-vetta, sei ‘guru’ haya
A person may be a brahmana or a sudra or a sannyasi, but if he happens to be well versed in the science of Krsna, then he is fit to become a spiritual master. (Cc. Madhya 8.128) The qualification, then, of a spiritual master is not to be a qualified brahmana, but to be well versed in the science of Krsna.
One who is conversant with Vedic wisdom is a brahmana. And only a brahmana who is a pure Vaisnava and knows all the intricacies of the science of Krsna can become a spiritual master.