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CC Madhya 8.128

kibā vipra, kibā nyāsī, śūdra kene naya
yei kṛṣṇa-tattva-vettā, sei ‘guru’ haya
Synonyms: 
kibā — whether; vipra — a brāhmaṇa; kibā — whether; nyāsī — a sannyāsī; śūdra — a śūdra; kene — why; naya — not; yei — anyone who; kṛṣṇa-tattva-vettā — a knower of the science of Kṛṣṇa; sei — that person; guru — the spiritual master; haya — is.
Translation: 
“Whether one is a brāhmaṇa, a sannyāsī or a śūdra — regardless of what he is — he can become a spiritual master if he knows the science of Kṛṣṇa.”
Purport: 

This verse is very important to the Kṛṣṇa consciousness movement. In his Amṛta-pravāha-bhāṣya, Śrīla Bhaktivinoda Ṭhākura explains that one should not think that because Śrī Caitanya Mahāprabhu was born a brāhmaṇa and was situated in the topmost spiritual order as a sannyāsī, it was improper for Him to receive instructions from Śrīla Rāmānanda Rāya, who belonged to the śūdra caste. To clarify this matter, Śrī Caitanya Mahāprabhu informed Rāmānanda Rāya that knowledge of Kṛṣṇa consciousness is more important than caste. In the system of varṇāśrama-dharma there are various duties for the brāhmaṇas, kṣatriyas, vaiśyas and śūdras. Actually the brāhmaṇa is supposed to be the spiritual master of all other varṇas, or classes, but as far as Kṛṣṇa consciousness is concerned, everyone is capable of becoming a spiritual master because knowledge in Kṛṣṇa consciousness is on the platform of the spirit soul. To spread Kṛṣṇa consciousness, one need only be cognizant of the science of the spirit soul. It does not matter whether one is a brāhmaṇa, kṣatriya, vaiśya, śūdra, sannyāsī, gṛhastha or whatever. If one simply understands this science, he can become a spiritual master.

It is stated in the Hari-bhakti-vilāsa that one should not accept initiation from a person who is not in the brahminical order if there is a fit person in the brahminical order present. This instruction is meant for those who are overly dependent on the mundane social order and is suitable for those who want to remain in mundane life. If one understands the truth of Kṛṣṇa consciousness and seriously desires to attain transcendental knowledge for the perfection of life, he can accept a spiritual master from any social status, provided the spiritual master is fully conversant with the science of Kṛṣṇa. Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura also states that although one is situated as a brāhmaṇa, kṣatriya, vaiśya, śūdra, brahmacārī, vānaprastha, gṛhastha or sannyāsī, if he is conversant in the science of Kṛṣṇa he can become a spiritual master as vartma-pradarśaka-guru, dīkṣā-guru or śikṣā-guru. The spiritual master who first gives information about spiritual life is called the vartma-pradarśaka-guru, the spiritual master who initiates according to the regulations of the śāstras is called the dīkṣā-guru, and the spiritual master who gives instructions for elevation is called the śikṣā-guru. Factually the qualifications of a spiritual master depend on his knowledge of the science of Kṛṣṇa. It does not matter whether he is a brāhmaṇa, kṣatriya, sannyāsī or śūdra. This injunction given by Śrī Caitanya Mahāprabhu is not at all against the injunctions of the śāstras. In the Padma Purāṇa it is said:

na śūdrā bhagavad-bhaktāste ’pi bhāgavatottamāḥ
sarva-varṇeṣu te śūdrā
ye na bhaktā janārdane

One who is actually advanced in spiritual knowledge of Kṛṣṇa is never a śūdra, even though he may have been born in a śūdra family. However, even if a vipra, or brāhmaṇa, is very expert in the six brahminical activities (paṭhana, pāṭhana, yajana, yājana, dāna, pratigraha) and is also well versed in the Vedic hymns, he cannot become a spiritual master unless he is a Vaiṣṇava. But if one is born in the family of caṇḍālas yet is well versed in Kṛṣṇa consciousness, he can become a guru. These are the śāstric injunctions, and strictly following these injunctions, Śrī Caitanya Mahāprabhu, as a gṛhastha named Śrī Viśvambhara, was initiated by a sannyāsī-guru named Īśvara Purī. Similarly, Śrī Nityānanda Prabhu was initiated by Mādhavendra Purī, a sannyāsī. According to others, however, He was initiated by Lakṣmīpati Tīrtha. Advaita Ācārya, although a gṛhastha, was initiated by Mādhavendra Purī, and Śrī Rasikānanda, although born in a brāhmaṇa family, was initiated by Śrī Śyāmānanda Prabhu, who was not born in a caste brāhmaṇa family. There are many instances in which a born brāhmaṇa took initiation from a person who was not born in a brāhmaṇa family. The brahminical symptoms are explained in Śrīmad-Bhāgavatam (7.11.35), wherein it is stated:

yasya yal-lakṣaṇaṁ proktaṁpuṁso varṇābhivyañjakam
yad anyatrāpi dṛśyeta
tat tenaiva vinirdiśet

If a person is born in a śūdra family but has all the qualities of a spiritual master, he should be accepted not only as a brāhmaṇa but as a qualified spiritual master also. This is also the instruction of Śrī Caitanya Mahāprabhu. Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura therefore introduced the sacred thread ceremony for all Vaiṣṇavas according to the rules and regulations.

Sometimes a Vaiṣṇava who is a bhajanānandī does not take the sāvitra-saṁskāra (sacred thread initiation), but this does not mean that this system should be used for preaching work. There are two kinds of Vaiṣṇavas — bhajanānandī and goṣṭhy-ānandī. A bhajanānandī is not interested in preaching work, but a goṣṭhy-ānandī is interested in spreading Kṛṣṇa consciousness to benefit the people and increase the number of Vaiṣṇavas. A Vaiṣṇava is understood to be above the position of a brāhmaṇa. As a preacher, he should be recognized as a brāhmaṇa; otherwise there may be a misunderstanding of his position as a Vaiṣṇava. However, a Vaiṣṇava brāhmaṇa is not selected on the basis of his birth but according to his qualities. Unfortunately, those who are unintelligent do not know the difference between a brāhmaṇa and a Vaiṣṇava. They are under the impression that unless one is a brāhmaṇa he cannot be a spiritual master. For this reason only, Śrī Caitanya Mahāprabhu makes the statement in this verse:

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

If one becomes a guru, he is automatically a brāhmaṇa. Sometimes a caste guru says that ye kṛṣṇa-tattva-vettā, sei guru haya means that one who is not a brāhmaṇa may become a śikṣā-guru or a vartma-pradarśaka-guru but not an initiator guru. According to such caste gurus, birth and family ties are considered foremost. However, the hereditary consideration is not acceptable to Vaiṣṇavas. The word guru is equally applicable to the vartma-pradarśaka-guru, śikṣā-guru and dīkṣā-guru. Unless we accept the principle enunciated by Śrī Caitanya Mahāprabhu, this Kṛṣṇa consciousness movement cannot spread all over the world. According to Śrī Caitanya Mahāprabhu’s intentions, pṛthivīte āche yata nagarādi-grāma sarvatra pracāra haibe mora nāma. Śrī Caitanya Mahāprabhu’s cult must be preached all over the world. This does not mean that people should take to His teachings and remain śūdras or caṇḍālas. As soon as one is trained as a pure Vaiṣṇava, he must be accepted as a bona fide brāhmaṇa. This is the essence of Śrī Caitanya Mahāprabhu’s instructions in this verse.