By the mercy of Krsna one finds a bonafide spiritual master
and by the mercy of the spiritual master, one finds Krsna.
"The spiritual master is receiving benediction from the ocean of mercy. Just as a cloud pours water on a forest fire to extinguish it, so the spiritual master delivers the materially afflicted world by extinguishing the blazing fire of material existence. I offer my respectful obeisances unto the lotus feet of such a spiritual master, who is an ocean of auspicious qualities."
Sri Gurvastakam, Verse 1
"....It has been established that Brahmaji, the firstborn living entity, received knowledge directly from the Supreme Lord, and the same knowledge was imparted to Narada, the next disciple. Narada asked to receive the knowledge, and Brahmaji imparted it upon being asked. Therefore, asking for transcendental knowledge from the right person and receiving it properly is the regulation of the disciplic succession. This process is recommended in the Bhagavad-gita (4.2). The inquisitive student must approach a qualified spiritual master to receive transcendental knowledge by surrender, submissive inquiries and service. Knowledge received by submissive inquiries and service is more effective than knowledge received in exchange for money. A spiritual master in the line of disciplic succession from Brahma and Narada has no demand for dollars and cents. A bona fide student has to satisfy him by sincere service to obtain knowledge of the relation and nature of the individual soul and the Supersoul."
Srimad-Bhagavatam 2:5:1 Purport
"One should approach a guru who can extinguish the blazing fire of this material world, the struggle for existence. People want to be cheated, and therefore they go to yogis and svamis who play tricks, but tricks do not mitigate the miseries of material life. If being able to manufacture gold is a criterion for becoming God, then why not accept Krsna, the proprietor of the entire universe, wherein there are countless tons of gold? As mentioned before, the color of gold is compared to the will-O'-the-wisp or yellow stool; therefore one should not be allured by gold-manufacturing gurus but should sincerely approach a devotee like Jada Bharata. Jada Bharata instructed Rahugana Maharaja so well that the King was relieved from the bodily conception. One cannot become happy by accepting a false guru. A guru should be accepted as advised in Srimad-Bhagavatam (11.3.21). Tasmad gurum prapadyeta jijnasuh sreya uttamam: One should approach a bona fide guru to inquire about the highest benefit of life. Such a guru is described as follows: sabde pare ca nisnatam. Such a guru does not manufacture gold or juggle words. He is well versed in the conclusions of Vedic knowledge (vedais ca sarvair aham eva vedyah). He is freed from all material contamination and is fully engaged in Krsna's service. If one is able to obtain the dust of the lotus feet of such a guru, his life becomes successful. Otherwise he is baffled both in this life and in the next."
Srimad-Bhagavatam 5:14:13 Purport
"One's first duty is to accept a bona fide spiritual master. The student or disciple should be very inquisitive; he should be eager to know the complete truth about eternal religion (sanatana-dharma). The words guru-susrusaya mean that one should personally serve the spiritual master by giving him bodily comforts, helping him in bathing, dressing, sleeping, eating and so on. This is called guru-susrusanam. A disciple should serve the spiritual master as a menial servant, and whatever he has in his possession should be dedicated to the spiritual master. pranair arthair dhiya vaca. Everyone has his life, his wealth, his intelligence and his words, and all of them should be offered to the Supreme Personality of Godhead through the via medium of the spiritual master. Everything should be offered to the spiritual master as a matter of duty, but the offering should be made to the spiritual master with heart and soul, not artificially to gain material prestige. This offering is called arpana. Moreover, one should live among devotees, saintly persons, to learn the etiquette and proper behavior of devotional service. Srila Visvanatha Cakravarti Thakura remarks in this connection that whatever is offered to the spiritual master should be offered with love and affection, not for material adoration."
Srimad-Bhagavatam 7:7:30 Purport
"According to sastra, the duty of the guru is to take the disciple back home, back to Godhead. If he is unable to do so and instead hinders the disciple in going back to Godhead, he should not be a guru. Gurur na sa syat (Bhag. 5.5.18). One should not become a guru if he cannot enable his disciple to advance in Krsna consciousness. The goal of life is to become a devotee of Lord Krsna so that one may be freed from the bondage of material existence (tyaktva deham punar janma naiti mam eti so 'rjuna). The spiritual master helps the disciple attain this stage by developing Krsna consciousness. ....Anyone who is supposed to be a guru but who goes against the principle of visnu-bhakti cannot be accepted as guru. If one has falsely accepted such a guru, one should reject him."
Srimad-Bhagavatam 8:20:1 Purport
"....A materialistic so-called guru instructs his materialistic disciples about economic development and sense gratification, and because of such instructions the foolish disciples continue in the materialistic existence of ignorance. But Your Lordship gives knowledge that is eternal, and the intelligent person receiving such knowledge is quickly situated in his original constitutional position.
So-called gurus instruct their disciples for the sake of material profit. Some guru advises that one meditate in such a way that his intelligence will increase in regard to keeping his body fit for sense gratification. Another guru advises that sex is the ultimate goal of life and that one should therefore engage in sex to the best of his ability. These are the instructions of foolish gurus. In other words, because of the instructions of a foolish guru one remains perpetually in material existence and suffers its tribulations. But if one is intelligent enough to take instructions from the Supreme Personality of Godhead, as enunciated in Bhagavad-gita or the Sankhya philosophy of Kapiladeva, one can very soon attain liberation and be situated in his original position of spiritual life.
Srimad-Bhagavatam 8:24:50 Text and Purport
Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.