Book knowledge without realization of the Supreme Truth is useless. This is stated as follows:
ataḥ śrī-kṛṣṇa-nāmādi
na bhaved grāhyam indriyaiḥ
sevonmukhe hi jihvādau
svayam eva sphuraty adaḥ
(CC.2:17:136)
"No one can understand the transcendental nature of the name, form, quality and pastimes of Śrī Kṛṣṇa through his materially contaminated senses. Only when one becomes spiritually saturated by transcendental service to the Lord are the transcendental name, form, quality and pastimes of the Lord revealed to him." (Bhakti-rasāmṛta-sindhu 1.2.234)
This Bhagavad-gītā is the science of Kṛṣṇa consciousness. No one can become Kṛṣṇa conscious simply by mundane scholarship. One must be fortunate enough to associate with a person who is in pure consciousness. A Kṛṣṇa conscious person has realized knowledge, by the grace of Kṛṣṇa, because he is satisfied with pure devotional service. By realized knowledge, one becomes perfect. By transcendental knowledge one can remain steady in his convictions, but by mere academic knowledge one can be easily deluded and confused by apparent contradictions. It is the realized soul who is actually self-controlled, because he is surrendered to Kṛṣṇa. He is transcendental because he has nothing to do with mundane scholarship. For him mundane scholarship and mental speculation, which may be as good as gold to others, are of no greater value than pebbles or stones.
(Bhagavad-Gita----6:8----purport).

Herein we find the history of the Bhagavad-gītā traced from a remote time when it was delivered to the royal order of all planets, beginning from the sun planet. The kings of all planets are especially meant for the protection of the inhabitants, and therefore the royal order should understand the science of Bhagavad-gītā in order to be able to rule the citizens and protect them from material bondage to lust. Human life is meant for cultivation of spiritual knowledge, in eternal relationship with the Supreme Personality of Godhead, and the executive heads of all states and all planets are obliged to impart this lesson to the citizens by education, culture and devotion. In other words, the executive heads of all states are intended to spread the science of Kṛṣṇa consciousness so that the people may take advantage of this great science and pursue a successful path, utilizing the opportunity of the human form of life.
(Bhagavad-Gita----4:1----purport).

Although Arjuna was putting forward so many arguments based on knowledge of the principles of religion and moral codes, it appears that he was unable to solve his real problem without the help of the spiritual master, Lord Śrī Kṛṣṇa. He could understand that his so-called knowledge was useless in driving away his problems, which were drying up his whole existence; and it was impossible for him to solve such perplexities without the help of a spiritual master like Lord Kṛṣṇa. Academic knowledge, scholarship, high position, etc., are all useless in solving the problems of life; help can be given only by a spiritual master like Kṛṣṇa. Therefore, the conclusion is that a spiritual master who is one hundred percent Kṛṣṇa conscious is the bona fide spiritual master, for he can solve the problems of life. Lord Caitanya said that one who is master in the science of Kṛṣṇa consciousness, regardless of his social position, is the real spiritual master.
kibā vipra, kibā nyāsī, śūdra kene naya
yei kṛṣṇa-tattva-vettā, sei 'guru' haya
"It does not matter whether a person is a vipra (learned scholar in Vedic wisdom) or is born in a lower family, or is in the renounced order of life-if he is master in the science of Kṛṣṇa, he is the perfect and bona fide spiritual master" (CC.2:8:128). So without being a master in the science of Kṛṣṇa consciousness, no one is a bona fide spiritual master. It is also said in Vedic literature:
ṣaṭ-karma-nipuṇo vipro
mantra-tantra-viśāradaḥ
avaiṣṇavo gurur na syād
vaiṣṇavaḥ śva-paco guruḥ
"A scholarly brāhmaṇa, expert in all subjects of Vedic knowledge, is unfit to become a spiritual master without being a Vaiṣṇava, or expert in the science of Kṛṣṇa consciousness. But a person born in a family of a lower caste can become a spiritual master if he is a Vaiṣṇava, or Kṛṣṇa conscious." (Padma Purāṇa)
(Bhagavad-Gita----2:8----purport).

Tuṣṭi, satisfaction, means that one should not be eager to gather more and more material goods by unnecessary activity. One should be satisfied with whatever is obtained by the grace of the Supreme Lord; that is called satisfaction. Tapas means austerity or penance. There are many rules and regulations in the Vedas which apply here, like rising early in the morning and taking a bath. Sometimes it is very troublesome to rise early in the morning, but whatever voluntary trouble one may suffer in this way is called penance. Similarly, there are prescriptions for fasting on certain days of the month. One may not be inclined to practice such fasting, but because of his determination to make advancement in the science of Kṛṣṇa consciousness, he should accept such bodily troubles when they are recommended. However, one should not fast unnecessarily or against Vedic injunctions. One should not fast for some political purpose; that is described in Bhagavad-gītā as fasting in ignorance, and anything done in ignorance or passion does not lead to spiritual advancement. Everything done in the mode of goodness does advance one, however, and fasting done in terms of the Vedic injunctions enriches one in spiritual knowledge.
(Bhagavad-Gita----10:4:5----purport).

There are different manifestations of the Lord. He is one, but He has become many. He divides Himself into two different expansions, one called kalā and the other vibhinnāṁśa. Ordinary living entities are called vibhinnāṁśa expansions, and the unlimited expansions of viṣṇu-tattva, such as Vāmana, Govinda, Nārāyaṇa, Pradyumna, Vāsudeva and Ananta, are called svāṁśa-kalā. Svāṁśa refers to a direct expansion, and kalā denotes an expansion from the expansion of the original Lord. Baladeva is an expansion of Kṛṣṇa, and from Baladeva the next expansion is Saṅkarṣaṇa; thus Saṅkarṣaṇa is kalā, but Baladeva is svāṁśa. There is no difference, however, among Them. This is very nicely explained in the Brahma-saṁhitā (5.46): dīpārcir eva hi daśāntaram abhyupetya. With one candle one may light a second candle, with the second a third and then a fourth, and in this way one can light up thousands of candles, and no candle is inferior to another in distributing light. Every candle has the full potential candlepower, but there is still the distinction that one candle is the first, another the second, another the third and another the fourth. Similarly, there is no difference between the immediate expansion of the Lord and His secondary expansion. The Lord's names are considered in exactly the same way; since the Lord is absolute, His name, His form, His pastimes, His paraphernalia and His quality all have the same potency. In the absolute world, the name Kṛṣṇa is the transcendental sound representation of the Lord. There is no potential difference between His quality, name, form, etc. If we chant the name of the Lord, Hare Kṛṣṇa, that has as much potency as the Lord Himself. There is no potential difference between the form of the Lord whom we worship and the form of the Lord in the temple. One should not think that one is worshiping a doll or statue of the Lord, even if others consider it to be a statue. Because there is not potential difference, one gets the same result by worshiping the statue of the Lord or the Lord Himself. This is the science of Kṛṣṇa consciousness.
(Srimad Bhagavatam----3:21:32----purport).

The Lord says that those who are advanced in transcendental knowledge and know the science of Kṛṣṇa consciousness are very dear to Him, and He also is very dear to them. Those who are actually in perfect knowledge of the science of God know that the living entities are superior energy of the Supreme Lord. This is stated in Bhagavad-gītā, Seventh Chapter: the material energy is inferior, and the living entities are superior energy. Energy and the energetic are nondifferent; therefore, energies possess the same quality as the energetic. Persons who are in full knowledge of the Personality of Godhead, analyzing His different energies and knowing their own constitutional position, are certainly very, very dear to the Lord. Persons, however, who may not even be conversant with knowledge of the Supreme Personality but who always think of the Lord with love and faith, feeling that He is great and that they are His parts and parcels, ever His servitors, are even more favored by Him.
(Srimad Bhagavatam----4:7:38----purport).

A devotee is one who has no other motive for reciting Bhagavad-gītā and Śrīmad-Bhāgavatam than to serve the Lord. Śrī Caitanya Mahāprabhu has therefore advised us to hear the glories of the Lord from a realized person (bhāgavata paro diya bhāgavata sthane). Unless one is personally a realized soul in the science of Kṛṣṇa consciousness, a neophyte should not approach him to hear about the Lord, 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
One should avoid hearing from a person not situated in Vaiṣṇava behavior. A Vaiṣṇava is nivṛtta-tṛṣṇa; that is, he has no material purpose, for his only purpose is to preach Kṛṣṇa consciousness. So-called scholars, philosophers and politicians exploit the importance of Bhagavad-gītā by distorting its meaning for their own purposes. Therefore this verse warns that kṛṣṇa-kathā should be recited by a person who is nivṛtta-tṛṣṇa.
(Srimad Bhagavatam----10:1:4----purport).

A child can be stopped from engaging in nonsense by being engaged in superior activities. The forceful restraint of sense activities by the eight principles of yoga is recommended for inferior men. Being engaged in the superior activities of Kṛṣṇa consciousness, superior men naturally retire from the inferior activities of material existence. In this way Lord Caitanya teaches the science of Kṛṣṇa consciousness. That science is absolute. Dry mental speculators try to restrain themselves from material attachment, but it is generally found that the mind is too strong to be controlled and that it drags them down to sensual activities. A person in Kṛṣṇa consciousness does not run this risk. One has to engage his mind and senses in Kṛṣṇa conscious activities, and Lord Caitanya teaches one how to do this in practice.
(Teachings of Lord Caitanya).

The lower stratum of those who strive for perfection are the karmīs, or fruitive workers, who look to gratify their senses. Above them are the jñānīs, or seekers of knowledge, who restrain the urges of their senses and become situated on the subtle, mental plane. Superior to them are the yogīs who seek mystic perfection. Lord Caitanya has described all these persons as aśānta, restless. Among them, those who are free of all material designations and are rid of the false ego, and who are thus liberated beings situated in the self—they alone can understand Lord Kṛṣṇa, the Supreme Person, in truth. When they become fully conversant with the science of Kṛṣṇa consciousness, such saintly souls can act as spiritual masters for all humanity, regardless of any external designation.
(Renunciation Through Wisdom).

Although the companions of Lord Kṛṣṇa and Balarāma were simple cowherd boys, they were in a position to dictate even to the high-class brāhmaṇas engaged in the Vedic rituals of sacrifice. But the smārta-brāhmaṇas, who were simply sacrificial-minded, could not understand the dictation of the transcendental devotees of the Lord. They could not even appreciate the begging of the Supreme Lord, Kṛṣṇa and Balarāma. Although they heard all the arguments on behalf of Kṛṣṇa and Balarāma, they did not care for them, and they refused to speak to the boys. Despite being highly elevated in the knowledge of Vedic sacrificial rites, all such nondevotee brāhmaṇas, although they think of themselves as very highly elevated, are ignorant, foolish persons. All their activities are childish because they do not know the purpose of the Vedas, as it is explained in the Bhagavad-gītā: to understand Kṛṣṇa. In spite of their advancement in Vedic knowledge and rituals, they do not understand Kṛṣṇa; therefore their knowledge of the Vedas is useless. Lord Caitanya, therefore, gave His valuable opinion that although a person may not be born in a brāhmaṇa family, if he knows Kṛṣṇa or the science of Kṛṣṇa consciousness he is more than a brāhmaṇa, and he is quite fit to become a spiritual master.
(Krsna Book).