(Click to enlarge photo of His Divine Grace A.C. Bhaktivedanta Swami Prabhupada)
"The Supreme Personality of Godhead is actually the supreme spiritual master. The Supreme Lord knows everything about the suffering of the conditioned soul, and therefore He appears in this material world, sometimes personally, sometimes by an incarnation and sometimes by authorizing a living being to act on His behalf. In all cases, however, He is the original spiritual master who enlightens the conditioned souls who are suffering in the material world. The Lord is always busy helping the conditioned souls in many ways. Therefore He is addressed here as paramo gurur bhavān. The representative of the Supreme Personality of Godhead who acts to spread Kṛṣṇa consciousness is also guided by the Supreme Lord to act properly in executing the Lord's order. Such a person may appear to be an ordinary human being, but because he acts on behalf of the Supreme Personality of Godhead, the supreme spiritual master, he is not to be neglected as ordinary. It is therefore said, ācāryaṁ māṁ vijānīyāt (SB 11.17.27): an ācārya who acts on behalf of the Supreme Personality of Godhead should be understood to be as good as the Supreme Lord Himself.
sākṣād dharitvena samasta-śāstrair
uktas tathā bhāvyata eva sadbhiḥ
kintu prabhor yaḥ priya eva tasya
vande guroḥ śrī-caraṇāravindam
Viśvanātha Cakravartī Ṭhākura has advised that the spiritual master acting on the Supreme Lord's behalf must be worshiped as being as good as the Supreme Lord, for he is the Lord's most confidential servant in broadcasting the Lord's message for the benefit of the conditioned souls involved in the material world."
(Śrīmad-Bhāgavatam 8.24.46, Purport)
"An authorized spiritual master is as good as Hari, the Supreme Personality of Godhead. As Hari is free to act as He likes, the empowered spiritual master is also free. As Hari is not subject to mundane rules and regulations, the spiritual master empowered by Him is also not subject. According to the Caitanya-caritāmṛta (Antya-līlā 7.11), kṛṣṇa-śakti vinā nahe tāra pravartana. An authorized spiritual master empowered by Kṛṣṇa can spread the glories of the holy name of the Lord, for he has power of attorney from the Supreme Personality of Godhead. In the mundane world, anyone possessing his master’s power of attorney can act on behalf of his master. Similarly, a spiritual master empowered by Kṛṣṇa through his own bona fide spiritual master should be considered as good as the Supreme Personality of Godhead Himself."
(Caitanya-caritāmṛta Madhya 10.136, Purport)
"In India there are many so-called gurus, and they are limited to a certain district or a province. They do not even travel about India, yet they declare themselves to be jagad-guru, the guru of the whole world. Such cheating gurus should not be accepted. Anyone can see how the bona fide spiritual master accepts disciples from all over the world. The guru is a qualified brāhmaṇa; therefore he knows Brahman and Parabrahman. He thus devotes his life for the service of Parabrahman. The bona fide spiritual master who accepts disciples from all over the world is also worshiped all over the world because of his qualities. Lokānām asau pūjyo yathā hariḥ: the people of the world worship him just as they worship the Supreme Personality of Godhead. All these honors are offered to him because he strictly follows the brahminical principles and teaches these principles to his disciples. Such a person is called an ācārya because he knows the principles of devotional service, he behaves in that way himself, and he teaches his disciples to follow in his footsteps. Thus he is an ācārya or jagad-guru."
(Caitanya-caritāmṛta Madhya 24.330, Purport)
"Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura explains that unless one is directly empowered by the causeless mercy of Kṛṣṇa, one cannot become the spiritual master of the entire world (jagad-guru). One cannot become an ācārya simply by mental speculation. The true ācārya presents Kṛṣṇa to everyone by preaching the holy name of the Lord throughout the world. Thus the conditioned souls, purified by chanting the holy name, are liberated from the blazing fire of material existence. In this way, spiritual benefit grows increasingly full, like the waxing moon in the sky. The true ācārya, the spiritual master of the entire world, must be considered an incarnation of Kṛṣṇa’s mercy. Indeed, he is personally embracing Kṛṣṇa. He is therefore the spiritual master of all the varṇas (brāhmaṇa, kṣatriya, vaiśya and śūdra) and all the āśramas (brahmacarya, gṛhastha, vānaprastha and sannyāsa). Since he is understood to be the most advanced devotee, he is called paramahaṁsa-ṭhākura. Ṭhākura is a title of honor offered to the paramahaṁsa. Therefore one who acts as an ācārya, directly presenting Lord Kṛṣṇa by spreading His name and fame, is also to be called paramahaṁsa-ṭhākura."
(Caitanya-caritāmṛta Antya 7-12, Purport)
"So far as your question about the disciplic succession, in all Vedic literatures it is mentioned about the disciplic succession. You have read in Srimad-Bhagavatam, first canto, first chapter where it is said that Brahma was inspired from within the heart by the Supreme Personality of Godhead. Similarly, in the Katha Upanisad it is said that 'in order to learn the transcendental science, one has to approach the Spiritual Master.' This spiritual master means one who has heard correctly from his Spiritual Master. In this way, ultimately it goes to Krishna, the Supreme Spiritual Master. There is no possibility of understanding the Vedic knowledge without this descending process. The modern world is puffed up with personal research knowledge, but the person who is engaging in this research work does not know that he is imperfect on account of his aptitude for becoming illusioned, for committing mistakes, for his cheating propensity and for his possessing imperfect senses. Therefore there is no possibility of receiving perfect knowledge without approaching a self-realized Spiritual Master coming down in disciplic succession. The mental speculator, no matter how advanced he may be, cannot deliver us the right knowledge."
(Srila Prabhupada Letter, February 28, 1969)
"Perhaps you know the vedic verse:
yasya deve para bhakti yatha deve tatha gurou
tasta ete kathita hi artha prakasante mahatmanah
'Anyone who has got full faith in the Supreme Personality of Godhead and similar faith in the Spiritual Master, to him the secret of Vedic knowledge becomes revealed.' The Supreme Spiritual Master is Krsna Himself, and the chain of disciplic succession is the dearmost confidential servant of Krsna. So whatever you are doing under my direction, it is going to my Spiritual Master and thus up to the Gosvamis and then to the Supreme Personality of Godhead, Lord Caitanya or Krsna."
(Srila Prabhupada Letter, June 14, 1970)
"Kṛṣṇa is so kind. He is guru within, and when we are serious, He manifests Himself as a spiritual master. This is described in the Caitanya-caritāmṛta. So guru is helping from inside and from outside. Antar-bahiḥ. So Kṛṣṇa is so kind. Therefore Kṛṣṇa manifests Himself as spiritual master. Therefore Viśvanātha Cakravartī Ṭhākura says, sākṣād-dhari, sākṣād-dharitvena samasta-śāstraiḥ. Manifestation of; empowered. One who is pure devotee, he's empowered by Kṛṣṇa to act as spiritual master. Therefore spiritual master should be accepted: sākṣād-dharitvena samasta-śāstrair uktaḥ. It is described there. Tathā bhāvyata eva sadbhiḥ. Those who are learned scholar, actual devotee, they accept. But that does not mean guru is Kṛṣṇa. Kintu prabhor yaḥ priya eva tasya. Now, that, if you accept, you... Accept means you honor spiritual master as good as Kṛṣṇa, because he's representative. But not that Māyāvāda philosophy that guru and Kṛṣṇa is the same. This is simultaneously one and different. Kintu prabhor yaḥ priya eva tasya. He is as good as Kṛṣṇa because he is very, very dear to Kṛṣṇa. Why dear? Because he's preaching on behalf of Kṛṣṇa. What Kṛṣṇa wants, he's doing that. Therefore he's very dear."
(Srila Prabhupada Lecture, Vrindavan, August 15, 1974)