SB 8.24.48
yat-sevayagner iva rudra-rodanam
puman vijahyan malam atmanas tamah
bhajeta varnam nijam esa so ’vyayo
bhuyat sa isah paramo guror guruh
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
One who wants to be free of material entanglement should take to the service of the Supreme Personality of Godhead and give up the contamination of ignorance, involving pious and impious activities. Thus one regains his original identity, just as a block of gold or silver sheds all dirt and becomes purified when treated with fire. May that inexhaustible Supreme Personality of Godhead become our spiritual master, for He is the original spiritual master of all other spiritual masters.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
In human life one is meant to undergo austerity to purify one’s existence. Tapo divyam putraka yena sattvam suddhyet. Because of contamination by the modes of material nature, one continues in the cycle of birth and death (karanam guna-sango ’sya sad-asad-yoni janmasu). Therefore the purpose of human life is to purify oneself of this contamination so that one can regain his spiritual form and not undergo this cycle of birth and death. The recommended process of decontamination is devotional service to the Lord. There are various processes for self-realization, such as karma, jñana and yoga, but none of them is equal to the process of devotional service. As gold and silver can be freed from all dirty contamination by being put into a fire but not merely by being washed, the living entity can be awakened to his own identity by performing devotional service (yat-sevaya), but not by karma, jñana or yoga. Cultivation of speculative knowledge or practice of yogic gymnastics will not be helpful.
The word varnam refers to the luster of one’s original identity. The original luster of gold or silver is brilliant. Similarly, the original luster of the living being, who is part of the sac-cid-ananda-vigraha, is the luster of ananda, or pleasure. Anandamayo bhyasat. Every living entity has the right to become anandamaya, joyful, because he is part of the sac-cid-ananda-vigraha, Krsna. Why should the living being be put into tribulation because of dirty contamination by the material modes of nature? The living entity should become purified and regain his svarupa, his original identity. This he can do only by devotional service. Therefore, one should adopt the instructions of the Supreme Personality of Godhead, who is described here as guror guruh, the spiritual master of all other spiritual masters.
Even though we may not have the fortune to contact the Supreme Lord personally, the Lord’s representative is as good as the Lord Himself because such a representative does not say anything unless it is spoken by the Supreme Personality of Godhead. Sri Caitanya Mahaprabhu therefore gives a definition of guru. Yare dekha, tare kaha ‘krsna’-upadesa: the bona fide guru is he who advises his disciples exactly in accordance with the principles spoken by Krsna. The bona fide guru is he who has accepted Krsna as guru. This is the guru-parampara system. The original guru is Vyasadeva because he is the speaker of Bhagavad-gita and Srimad-Bhagavatam, wherein everything spoken relates to Krsna. Therefore guru-puja is known as Vyasa-puja. In the final analysis, the original guru is Krsna, His disciple is Narada, whose disciple is Vyasa, and in this way we gradually come in touch with the guru-parampara. One cannot become a guru if he does not know what the Personality of Godhead Krsna or His incarnation wants. The mission of the guru is the mission of the Supreme Personality of Godhead: to spread Krsna consciousness all over the world.