The next symptom of the age of Kali is that principles of religion, which are all spotlessly white, like the white lotus flower, will be attacked by the uncultured śūdra population of the age. They may be descendants of brāhmaṇa or kṣatriya forefathers, but in the age of Kali, for want of sufficient education and culture of Vedic wisdom, such a śūdra-like population will defy the principles of religion, and persons who are religiously endowed will be terrified by such men. They will declare themselves as adherents of no religious principles, and many "isms" and cults will spring up in Kali-yuga only to kill the spotless bull of religion. The state will be declared to be secular, or without any particular principle of religion, and as a result there will be total indifference to the principles of religion. The citizens will be free to act as they like, without respect for sādhu, śāstra and guru. The bull standing on one leg indicates that the principles of religion are gradually diminishing. Even the fragmental existence of religious principles will be embarrassed by so many obstacles as if in the trembling condition of falling down at any time.
(Srimad Bhagavatam----1:17:2----purport).

Although Mahārāja Pṛthu was simply enthroned on the royal seat and was not at that time exhibiting his godly powers, the reciters like the sūta, the māgadha and the vandī understood that King Pṛthu was an incarnation of God. They could understand this by the instructions given by the great sages and learned brāhmaṇas. We have to understand the incarnations of God by the instructions of authorized persons. We cannot manufacture a God by our own concoctions. As stated by Narottama dāsa Ṭhākura, sādhu-śāstra-guru: one has to test all spiritual matters according to the instructions of saintly persons, scriptures and the spiritual master. The spiritual master is one who follows the instructions of his predecessors, namely the sādhus, or saintly persons. A bona fide spiritual master does not mention anything not mentioned in the authorized scriptures. Ordinary people have to follow the instructions ofsādhu, śāstra and guru. Those statements made in the śāstras and those made by the bona fide sādhu or guru cannot differ from one another.
(Srimad Bhagavatam----4:16:41----purport).

Narottama dāsa Ṭhākura states that one has to ascertain the right path for his activities by following in the footsteps of great saintly persons and books of knowledge under the guidance of a spiritual master (sādhu-śāstra-guru-vākya). A saintly person is one who follows the Vedic injunctions, which are the orders of the Supreme Personality of Godhead. The word guru refers to one who gives proper direction under the authority of the Vedic injunctions and according to the examples of the lives of great personalities. The best way to mold one's life is to follow in the footsteps of the authorized personalities like those mentioned herein by Pṛthu Mahārāja, beginning with Svāyambhuva Manu. The safest path in life is to follow such great personalities, especially those mentioned in the Śrīmad-Bhāgavatam. The mahājanas, or great personalities, are Brahmā, Lord Śiva, Nārada Muni, Manu, the Kumāras, Prahlāda Mahārāja, Bali Mahārāja, Yamarāja, Bhīṣma, Janaka, Śukadeva Gosvāmī and Kapila Muni.
(Srimad Bhagavatam----4:21:28-29----purport).

When a man comes into good consciousness and accepts a saintly person as a spiritual master, he hears many Vedic instructions in the form of philosophy, stories, narrations about great devotees and transactions between God and His devotees. In this way a man becomes refreshed in mind, exactly like a person who smears scented sandalwood pulp all over his body and decorates himself with ornaments. These decorations may be compared to knowledge of religion and the self. Through such knowledge one becomes detached from a materialistic way of life and engages himself in always hearing Śrīmad-Bhāgavatam, Bhagavad-gītā and other Vedic literatures. The word sādhv-alaṅkṛta used in this verse indicates that one must be absorbed in knowledge gathered from the instructions of saintly persons. Just as King Purañjana began to search out his better half, the Queen, one who is decorated with knowledge and instructions from saintly persons should try to search out his original consciousness, Kṛṣṇa consciousness. One cannot return to Kṛṣṇa consciousness unless he is favored by the instructions of a saintly person. Therefore Śrīla Narottama dāsa Ṭhākura sings: sādhu-śāstra-guru-vākya, cittete kariyā aikya. If we want to become saintly persons, or if we want to return to our original Kṛṣṇa consciousness, we must associate with sādhu (a saintly person), śāstra (authoritative Vedic literature) and guru (a bona fide spiritual master). This is the process.
(Srimad Bhagavatam----4:26:12----purport).

It is herein stated that one should strictly follow the scriptures if one actually wants an auspicious life. The same is explained in Bhagavad-gītā (16.23):

yaḥ śāstra-vidhim utsṛjya
vartate kāma-kārataḥ
na sa siddhim avāpnoti
na sukhaṁ na parāṁ gatim
"He who discards scriptural injunctions and acts according to his own whims attains neither perfection, nor happiness, nor the supreme destination." One who does not strictly follow the terms of the Vedic injunctions never attains success in life or happiness. And what to speak of going home, back to Godhead.

One śāstric injunction holds that a householder, a kṣatriya or an administrative head should not refuse to accept a woman if she voluntarily requests to become a wife. Since Kālakanyā, the daughter of Time, was deputed by Nārada Muni to offer herself to Yavana-rāja, the King of the Yavanas could not refuse her. All transactions must be performed in light of the śāstric injunctions. The śāstric injunctions are confirmed by great sages like Nārada Muni. As stated by Narottama dāsa Ṭhākura: sādhu-śāstra-guru-vākya, cittete kariyā aikya. One should follow the principles of saintly persons, scriptures and the spiritual master. In this way one is sure to attain success in life. Kālakanyā, the daughter of Time, presented herself before the King of the Yavanas precisely in terms of sādhu, śāstra and guru. Thus there was no reason for not accepting her.
(Srimad Bhagavatam----4:27:26----purport).

One must accept the bona fide spiritual master and render service unto him with great devotion and faith. Whatever one has in one's possession should be offered to the spiritual master, and in the association of saintly persons and devotees one should worship the Lord, hear the glories of the Lord with faith, glorify the transcendental qualities and activities of the Lord, always meditate on the Lord's lotus feet, and worship the Deity of the Lord strictly according to the injunctions of the śāstra and guru.
(Srimad Bhagavatam----7:7:32----translation).

One should not give up the process of devotional service, which is performed in nine different ways (śravaṇaṁ kīrtanaṁ viṣṇoḥ smaraṇaṁ pāda-sevanam, etc. [SB 7.5.23]). The most important process is hearing (śravaṇam) from the guru, sādhu and śāstra-the spiritual master, the saintly ācāryas and the Vedic literature. Sādhu-śāstra-guru-vākya, cittete kariyā aikya. We should not hear the commentaries and explanations of nondevotees, for this is strictly forbidden by Śrīla Sanātana Gosvāmī, who quotes from the Padma Purāṇa:
avaiṣṇava-mukhodgīrṇaṁ
pūtaṁ hari-kathāmṛtam
śravaṇaṁ naiva kartavyaṁ
sarpocchiṣṭaṁ yathā payaḥ
We should strictly follow this injunction and never try to hear from Māyāvādīs, impersonalists, voidists, politicians or so-called scholars. Strictly avoiding such inauspicious association, we should simply hear from pure devotees. Śrīla Rūpa Gosvāmī therefore recommends, śrī-guru-padāśrayaḥ: one must seek shelter at the lotus feet of a pure devotee who can be one's guru. Caitanya Mahāprabhu advises that a guru is one who strictly follows the instructions of Bhagavad-gītā: yare dekha, tare kaha, 'kṛṣṇa'-upadeśa (Cc. Madhya 7.128). A juggler, a magician or one who speaks nonsense as an academic career is not a guru. Rather, a guru is one who presents Bhagavad-gītā, Kṛṣṇa's instructions, as it is. Śravaṇa is very important; one must hear from the Vaiṣṇava sādhu, guru and śāstra.
(Srimad Bhagavatam----10:2:37----purport).

The Supreme Personality of Godhead can appear before us in many forms, as he likes, but we must know the true facts: janma karma ca me divyam evaṁ yo vetti tattvataḥ (Bg. 4.9). By following the instructions of sādhu, guru and śāstra—the saintly persons, the spiritual master and the authoritative scriptures—one can understand Kṛṣṇa, and then one makes his life successful by returning home, back to Godhead.
(Srimad Bhagavatam----10:3:46----purport).

One who is not taught by a bona fide spiritual master cannot understand the Vedic literature. To emphasize this point, Lord Kṛṣṇa, while instructing Arjuna, clearly said that it was because Arjuna was His devotee and confidential friend that he could understand the mystery of the Bhagavad-gītā. It is to be concluded, therefore, that one who wants to understand the mystery of revealed scriptures must approach a bona fide spiritual master, hear from him very submissively and render service to him. Then the import of the scriptures will be revealed. It is stated in the Vedas (Śvetāśvatara Up. 6.23):
yasya deve parā bhaktir yathā deve tathā gurau
tasyaite kathitā hy arthā prakāśante mahātmanaḥ
[ŚU 6.23]
“The real import of the scriptures is revealed to one who has unflinching faith in both the Supreme Personality of Godhead and the spiritual master.” Śrīla Narottama dāsa Ṭhākura advises,sādhu-śāstra-guru-vākya, hṛdaye kariyā aikya. The meaning of this instruction is that one must consider the instructions of the sādhu, the revealed scriptures and the spiritual master in order to understand the real purpose of spiritual life. Neither a sādhu (saintly person or Vaiṣṇava) nor a bona fide spiritual master says anything that is beyond the scope of the sanction of the revealed scriptures. Thus the statements of the revealed scriptures correspond to those of the bona fide spiritual master and saintly persons. One must therefore act with reference to these three important sources of understanding.
(Sri Caitanya Caritamrta----1:7:49----purport).

Narottama dāsa Ṭhākura says, sādhu-śāstra-guru-vākya, cittete kariyā aikya: “One should accept as one’s guide the words of the sādhus, the śāstra and the guru.” A sādhu is a great personality like Śrī Caitanya Mahāprabhu, the śāstras are the injunctions of revealed scriptures, and the guru, or spiritual master, is one who confirms the scriptural injunctions. Accepting the guidance of these three is the actual way of following the great personalities (mahājanas) for real advancement in life (mahājano yena gataḥ sa panthāḥ [Cc. Madhya 17.186]). A man covered by illusion cannot understand the proper way; therefore Śrī Caitanya Mahāprabhu says, dharma-sthāpana-hetu sādhura vyavahāra: “The behavior of a devotee is the criterion for all other behavior.” Śrī Caitanya Mahāprabhu Himself followed the devotional principles and taught others to follow them. Purī-gosāñira ye ācaraṇa, sei dharma sāra. Śrī Caitanya Mahāprabhu personally followed the behavior of Mādhavendra Purī and advised others to follow his principles. Unfortunately, people have been attracted to the material body since time immemorial.
(Sri Caitanya Caritamrta----2:17:185----purport).

Śrī Caitanya Mahāprabhu’s first statement (kṛṣṇa kene daraśana dibe kali-kāle) refers to the scriptures. According to scripture, Kṛṣṇa appears in Dvāpara-yuga, but He never appears as Himself in Kali-yuga. Rather, He appears in Kali-yuga in a covered form. As stated in Śrīmad-Bhāgavatam (11.5.32), kṛṣṇa-varṇaṁ tviṣākṛṣṇaṁ sāṅgopāṅgāstra-pārṣadam. Kṛṣṇa appears in the Age of Kali in the garb of a devotee, Śrī Caitanya Mahāprabhu, who always associates with His internal soldiers—Śrī Advaita Prabhu, Śrī Nityānanda Prabhu, Śrīvāsa Prabhu and Gadādhara Prabhu. Although Balabhadra Bhaṭṭācārya was personally serving Lord Kṛṣṇa in His role as a devotee (Caitanya Mahāprabhu), he mistook Lord Kṛṣṇa for an ordinary man and an ordinary man for Lord Kṛṣṇa because he did not follow the rules set down by śāstra and guru.
(Sri Caitanya Caritamrta----2:18:101----purport).

Being forgetful of his real position, the conditioned soul may take help from śāstra, guru and the Supersoul within his heart. Kṛṣṇa is situated within everyone’s heart as the Supersoul. As stated in the Bhagavad-gītā (18.61):

īśvaraḥ sarva-bhūtānām hṛd-deśe ‘rjuna tiṣṭhati
bhrāmayan sarva-bhūtāni yantrārūḍhāni māyayā
“The Supreme Lord is situated in everyone’s heart, O Arjuna, and is directing the wanderings of all living entities, who are seated as on a machine made of the material energy.”

As the śaktyāveśa-avatāra Vyāsadeva, Kṛṣṇa teaches the conditioned soul through the Vedic literatures. Kṛṣṇa externally appears as the spiritual master and trains the conditioned soul to come to Kṛṣṇa consciousness. When his original Kṛṣṇa consciousness is revived, the conditioned soul is delivered from the material clutches. Thus a conditioned soul is always helped by the Supreme Personality of Godhead in three ways—by the scriptures, the spiritual master and the Supersoul within the heart. The Lord is the deliverer of the conditioned soul and is accepted as the Supreme Lord of all living entities. Kṛṣṇa says in the Bhagavad-gītā (18.66):

sarva-dharmān parityajya mām ekaṁ śaraṇaṁ vraja
ahaṁ tvāṁ sarva-pāpebhyo mokṣayiṣyāmi mā śucaḥ
“Abandon all varieties of religion and just surrender unto Me. I shall deliver you from all sinful reactions. Do not fear.” This same instruction is found throughout all Vedic literatures. Sādhu,śāstra and guru act as the representatives of Kṛṣṇa, and the Kṛṣṇa consciousness movement is also taking place all over the universe. Whoever takes advantage of this opportunity becomes liberated.
(Sri Caitanya Caritamrta----2:20:123----purport).

Śrī Caitanya Mahāprabhu replied, “As in other ages an incarnation is accepted according to the directions of the śāstras, in this Age of Kali an incarnation of God should be accepted in that way. According to Śrī Caitanya Mahāprabhu, this is the way an incarnation should be accepted. Śrīla Narottama dāsa Ṭhākura says, sādhu-śāstra-guru-vākya, cittete kariyā aikya. One should accept a thing as genuine by studying the words of saintly people, the spiritual master and the śāstra. The actual center is the śāstra, the revealed scripture. If a spiritual master does not speak according to the revealed scripture, he is not to be accepted. Similarly, if a saintly person does not speak according to the śāstra, he is not a saintly person. The śāstra is the center for all. Unfortunately, at the present moment, people do not refer to the śāstras; therefore they accept rascals as incarnations, and consequently they have made incarnations into a very cheap thing. Intelligent people who follow Śrī Caitanya Mahāprabhu’s instructions and the instructions of the ācārya, the bona fide spiritual master, will not accept a pretender as an incarnation of God. In Kali-yuga, the only incarnation is Śrī Caitanya Mahāprabhu.
(Sri Caitanya Caritamrta----2:20:352----translation and purport).