SB 4.29.51

sa vai priyatamaś cātmā
 yato na bhayam aṇv api
iti veda sa vai vidvān
 yo vidvān sa gurur hariḥ
Word for word: 
saḥ — He; vai — certainly; priya-tamaḥ — the most dear; ca — also; ātmā — Supersoul; yataḥ — from whom; na — never; bhayam — fear; aṇu — little; api — even; iti — thus; veda — (one who) knows; saḥ — he; vai — certainly; vidvān — educated; yaḥ — he who; vidvān — educated; saḥ — he; guruḥ — spiritual master; hariḥ — not different from the Lord.
Translation: 
One who is engaged in devotional service has not the least fear in material existence. This is because the Supreme Personality of Godhead is the Supersoul and friend of everyone. One who knows this secret is actually educated, and one thus educated can become the spiritual master of the world. One who is an actually bona fide spiritual master, representative of Kṛṣṇa, is not different from Kṛṣṇa.
Purport: 

Śrīla Viśvanātha Cakravartī Ṭhākura says, sākṣād-dharitvena samasta-śāstrair uktas tathā bhāvyata eva sadbhiḥ: the spiritual master is described in every scripture as the representative of the Supreme Personality of Godhead. The spiritual master is accepted as identical with the Supreme Personality of Godhead because he is the most confidential servant of the Lord (kintu prabhor yaḥ priya eva tasya). The purport is that both the Supersoul and the individual soul are very dear to everyone. Everyone loves himself, and when he becomes more advanced, he loves the Supersoul also. A person who is self-realized does not recommend the worship of anyone but the Supersoul. He knows that to worship the Supreme Personality of Godhead is easier than to worship various demigods under the influence of lust and the desire for material enjoyment. The devotee is therefore always engaged in the loving devotional service of the Lord. Such a person is a true guru. In Padma Purāṇa it is said:

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

“Even if a brāhmaṇa is very learned in Vedic scriptures and knows the six occupational duties of a brāhmaṇa, he cannot become a guru, or spiritual master, unless he is a devotee of the Supreme Personality of Godhead. However, if one is born in a family of dog-eaters but is a pure devotee of the Lord, he can become a spiritual master.” The conclusion is that one cannot become a spiritual master unless he is a pure devotee of the Lord. One who is a spiritual master in accordance with the above descriptions of devotional service is to be understood as the Supreme Personality of Godhead personally present. According to the words mentioned here (gurur hariḥ), consulting a bona fide spiritual master means consulting the Supreme Personality of Godhead personally. One should therefore take shelter of such a bona fide spiritual master. Success in life means accepting a spiritual master who knows Kṛṣṇa as the only supreme beloved personality. One should worship such a confidential devotee of the Lord.