One is not accepted as a bona fide student in this movement unless he promises to follow the four regulative principles: no animal-killing, no intoxication, no illicit sex and no gambling.
(Srimad Bhagavatam----4:26:5----purport).

A candidate for spiritual advancement must be nonviolent, must follow in the footsteps of great ācāryas, must always remember the nectar of the pastimes of the Supreme Personality of Godhead, must follow the regulative principles without material desire and, while following the regulative principles, should not blaspheme others. A devotee should lead a very simple life and not be disturbed by the duality of opposing elements. He should learn to tolerate them.
(Srimad Bhagavatam----4:22:24----purport).

By chanting the Hare Kṛṣṇa mahā-mantra regularly and following the regulative principles, one becomes qualified to be initiated as a brāhmaṇa.
(Srimad Bhagavatam----4:31:10----purport).

Śrīla Rūpa Gosvāmī has instructed in his Upadeśāmṛta that one should execute devotional service with great enthusiasm, patience and conviction and should follow the rules and regulations, keep oneself clean from material contamination and stay in the association of devotees. By following these six principles, one is sure to achieve the desired result of devotional service. For him who patiently follows the regulative principles of devotional service, the time will undoubtedly come when he will achieve the desired result.
(Krsna Book).

One can be gradually elevated to the standard of real knowledge, or Kṛṣṇa consciousness, by practicing austerity and celibacy (brahmacarya), by controlling the mind, by controlling the senses, by giving up one's possessions in charity, by being avowedly truthful, by keeping clean and by practicing yoga-āsanas. However, if one is fortunate enough to get the association of a pure devotee, he can easily surpass all the practices for controlling the mind by the mystic yoga process simply by following the regulative principles of Kṛṣṇa consciousness—refraining from illicit sex, meat-eating, intoxication and gambling—and by engaging in the service of the Supreme Lord under the direction of the bona fide spiritual master.
(Nectar of Instruction----purport).

One should always think of oneself as a servant of the servant of the ācāryas, and thinking this, one should live in the society of Vaiṣṇavas. However, if one thinks that he has become very mature and can live separate from the association of Vaiṣṇavas and thus gives up all the regulative principles due to offending a Vaiṣṇava, one's position becomes very dangerous.
(Sri Caitanya Caritamrta----2:19:156----purport).

The unscrupulous conditioned soul cannot rigidly follow the principles prohibiting illicit sex. intoxication, gambling and meat-eating. Thus he falls down and takes shelter of people who are like monkeys. In the Kṛṣṇa consciousness movement these monkey disciples, being unable to follow the strict regulative principles, sometimes fall down and try to form societies based on sex.
(Srimad Bhagavatam----5:14:30----purport).

In the material condition, all living entities are engaged in sense gratification, but when they associate with devotees who follow the regulative principles, they become purified and awakened to their original consciousness. They then attempt to satisfy the senses of Lord Kṛṣṇa and engage in His devotional service.
(Sri Caitanya Caritamrta----2:24:202----purport).

When love of God deteriorates into lust, it is very difficult to return to the normal condition. Nonetheless, Kṛṣṇa consciousness is so powerful that even a late beginner can become a lover of God by following the regulative principles of devotional service.
(Bhagavad-Gita----3:41----purport).

Raghunātha dāsa Gosvāmī followed the regulative principles so strictly and rigidly that they were compared to the lines on a stone. As such lines cannot be erased at any time, so the regulative principles observed by Śrī Raghunātha dāsa Gosvāmī could not be changed under any circumstances.
(Sri Caitanya Caritamrta----3:6:309----purport).

There are many regulative principles in the śāstras and directions given by the spiritual master. These regulative principles should act as servants of the basic principle—that is, one should always remember Kṛṣṇa and never forget Him.
(Sri Caitanya Caritamrta----2:22:113----purport).

One has to follow those rules and regulations, unattached to them, because practice of sense gratification under regulations may also lead one to go astray—as much as there is always the chance of an accident, even on the royal roads.
(Bhagavad-Gita----3:34----purport).

If after accepting the spiritual master and being initiated one does not follow the rules and regulations of devotional service, then he is again fallen. One must be very vigilant to remember that he is the part and parcel of the transcendental body of Kṛṣṇa, and that it is his duty as part and parcel to give service to the whole, or Kṛṣṇa. If we do not render service to Kṛṣṇa then again we fall down. In other words, simply becoming initiated does not elevate one to the position of a high-class brāhmaṇa. One also has to discharge the duties and follow the regulative principles very rigidly.
(Nectar of Devotion).