Prabhupāda: Anyone who is thinking that, "I shall make spiritual advancement without guru," he is wrongly thinking. It is not possible. Just like if you want to see a big man, you have to go through the servant of that man. You have to take permission. The servant may say, "Sir, such-and-such man wants to see you. He is very nice man, appears to be a good man." "Oh," the master says, "he is good man? All right, bring him." By the recommendation of the servant.
Therefore we sing daily, yasya prasādād bhagavat-prasādaḥ. By the mercy of guru, immediately we become... get the mercy of God. Yasya prasādād bhagavat-prasādo yasyāprasādān na gatiḥ kuto 'pi. Yasyāprasādāt. By... if guru is not satisfied, then na gatiḥ kuto 'pi. He has no gati. Gati means advancement. He has no advancement
Dhyāyaṁ stuvaṁs tasya yaśas tri-sandhyam. Therefore this Gurvaṣṭaka, saṁsāra-dāvānala, we have to sing. So God's incarnation, here it is said, "There are many incarnation." So guru is also incarnation of God, mercy incarnation of God. Guru means that... God is within you, caitya-guru, the guru, or the spiritual master, within your heart. Īśvaraḥ sarva-bhūtānāṁ hṛd-deśe 'rjuna tiṣṭhati (BG 18.61).
So this Paramātmā is also incarnation of God. And the same Paramātmā, when He comes before you, being very much merciful upon you, to teach you from outside, that is guru. Therefore it is said, sākṣād-dharitvena samasta-śāstraiḥ: guru is the mercy incarnation of God. Sākṣāt, direct; hari-tvena, he is Hari, God. Samasta-śāstraiḥ. It is not that somewhere it is stated, somewhere it is not stated. No. In all Vedic literature. Sākṣād-dharitvena samasta-śāstrair uktaḥ. Uktaḥ means "It is said." And tathā bhāvyata eva sadbhiḥ. Sadbhiḥ, those who are real devotees, they accept this: "Yes, guru is exactly representative of Kṛṣṇa, mercy representative."
But from the guru's side, a disciple may respect—may respect; must respect: it is not "may"—must respect guru as God. But guru should not say that "Now I have become God." Then he is immediately fallen. If guru says: "I am incarnation of God, so you simply worship me..." So many things are going on, Māyāvāda philosophy, "There is no difference between God..." But a real guru says that "I am servant of the servant of the servant of the servant of God." Gopī-bhartuḥ pada-kamalayor dāsa-dāsa-dāsānudāsaḥ (CC Madhya 13.80). Real guru will never claim, although he is given the honor of God, but he does not claim, never claims that he is God. He always claims that "I am the most fallen servant of God." This is the position.
(Srila Prabhupada Lecture, Los Angeles, October 1, 1972)