Guru means “one who is heavy with knowledge”, able to destroy the suffering of a disciple with spiritual knowledge. This is the Sanskrit definition. Because the transcendental realm is beyond our immediate vision, neccessarily we must therefore learn about it, initially, by hearing from those who have realized that realm. That is Guru.
Guru must of heard the transcendental knowledge of Godhead in disciplic succession of Guru’s, going back orginally to Lord Krishna Himself. If a guru is not transmitting spiritual knowledge, ultimately having it’s source in God or His direct representatives, the person is NOT a true Guru, they cannot be, neccessarily.
Below from the Sri Chaitanya Caritamrta of medieval Bengal, we find a text with commentary by the Vaisnava Acharyas, discussing the important and nature of the bona-fide spiritual master.
“Krishnadāsa Kavirāja Gosvāmī has composed this Sanskrit verse for the beginning of his book, and now he will explain it in detail. He offers his respectful obeisances to the six principles of the Absolute Truth. Gurūn is plural in number because anyone who gives spiritual instructions based on the revealed scriptures is accepted as a spiritual master.
The importance and neccessity of Guru is spoken of in the standard text of spiritual life, Bhagavad-Gita, By Lord Krishna.
“Just try to learn the truth by approaching a spiritual master. Inquire from him submissively and render service unto him. The self-realized souls can impart knowledge unto you because they have seen the truth.”
The path of spiritual realization is undoubtedly difficult. The Lord therefore advises us to approach a bona fide spiritual master in the line of disciplic succession from the Lord Himself. No one can be a bona fide spiritual master without following this principle of disciplic succession.
The Lord is the original spiritual master, and a person in the disciplic succession can convey the message of the Lord as it is to his disciple. No one can be spiritually realized by manufacturing his own process, as is the fashion of the foolish pretenders. The Bhāgavatam (6.3.19) says, dharmaṁ tu sākṣād bhagavat-praṇītam: the path of religion is directly enunciated by the Lord.
Therefore, mental speculation or dry arguments cannot help lead one to the right path. Nor by independent study of books of knowledge can one progress in spiritual life. One has to approach a bona fide spiritual master to receive the knowledge. Such a spiritual master should be accepted in full surrender, and one should serve the spiritual master like a menial servant, without false prestige. Satisfaction of the self-realized spiritual master is the secret of advancement in spiritual life.
Inquiries and submission constitute the proper combination for spiritual understanding. Unless there is submission and service, inquiries from the learned spiritual master will not be effective. One must be able to pass the test of the spiritual master, and when he sees the genuine desire of the disciple, he automatically blesses the disciple with genuine spiritual understanding. In this verse, both blind following and absurd inquiries are condemned.