SB 3.6.30

mukhato ’vartata brahma
 puruṣasya kurūdvaha
yas tūnmukhatvād varṇānāṁ
 mukhyo ’bhūd brāhmaṇo guruḥ
Word for word: 
mukhataḥ — from the mouth; avartata — generated; brahma — the Vedic wisdom; puruṣasya — of the virāṭ-puruṣa, the gigantic form; kuru-udvaha — O chief of the Kuru dynasty; yaḥ — who are; tu — due to; unmukhatvāt — inclined to; varṇānām — of the orders of society; mukhyaḥ — the chief; abhūt — so became; brāhmaṇaḥ — called the brāhmaṇas; guruḥ — the recognized teacher or spiritual master.
Translation: 
O chief of the Kuru dynasty, the Vedic wisdom became manifested from the mouth of the virāṭ, the gigantic form. Those who are inclined to this Vedic knowledge are called brāhmaṇas, and they are the natural teachers and spiritual masters of all the orders of society.
Purport: 

As confirmed in Bhagavad-gītā (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 brāhmaṇas, those who are situated on the arms are called kṣatriyas, those who are situated on the waist are called vaiśyas, and those who are situated on the legs are called śūdras. Everyone is situated in the body of the Supreme in His gigantic viśva-rūpa 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 brāhmaṇas, who are inclined to the Vedic wisdom. One who is not inclined to the Vedic wisdom but to mundane affairs cannot be called a brāhmaṇa, even if he is born of a brāhmaṇa family or father. To have a brāhmaṇa father does not qualify one as a brāhmaṇa. The main qualification of a brāhmaṇa 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 brāhmaṇa. 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 brāhmaṇa.

A real brāhmaṇa 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 Vedānta. One who is situated in the impersonal Brahman and has no information of the Supreme Personality of Godhead may become a brāhmaṇa, but he cannot become a spiritual master. It is said in the Padma Purāṇa:

ṣaṭ-karma-nipuṇo vipro
 mantra-tantra-viśāradaḥ
avaiṣṇavo gurur na syād
 vaiṣṇavaḥ śva-paco guruḥ

An impersonalist can become a qualified brāhmaṇa, but he cannot become a spiritual master unless and until he is promoted to the stage of a Vaiṣṇava, or a devotee of the Personality of Godhead. Lord Caitanya, the great authority of Vedic wisdom in the modern age, stated:

kibā vipra, kibā nyāsī, śūdra kene naya
 yei kṛṣṇa-tattva-vettā, sei ‘guru’ haya

A person may be a brāhmaṇa or a śūdra or a sannyāsī, but if he happens to be well versed in the science of Kṛṣṇa, 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 brāhmaṇa, but to be well versed in the science of Kṛṣṇa.

One who is conversant with Vedic wisdom is a brāhmaṇa. And only a brāhmaṇa who is a pure Vaiṣṇava and knows all the intricacies of the science of Kṛṣṇa can become a spiritual master.