yaḥ śāstra-vidhim utsṛjya
vartate kāma-kārataḥ
na sa siddhim avāpnoti
na sukhaṁ na parāṁ gatim

Synonyms

Translation

He who discards scriptural injunctions and acts according to his own whims attains neither perfection, nor happiness, nor the supreme destination.

Purport

As described before, the śāstra-vidhi, or the direction of the śāstra, is given to the different castes and orders of human society. Everyone is expected to follow these rules and regulations.  Shastra comes from the word astra meaning weapon and when one follows or sincerley attempts to follow the injunctions of śāstra, he becomes empowered to inject the potency within the śabdha pramāna into the hearts of others. 

tasmāc chāstraṁ pramāṇaṁ te
kāryākārya-vyavasthitau
jñātvā śāstra-vidhānoktaṁ
karma kartum ihārhasi

Translation

One should therefore understand what is duty and what is not duty by the regulations of the scriptures. Knowing such rules and regulations, one should act so that he may gradually be elevated.

Purport

As stated in the Fifteenth Chapter, all the rules and regulations of the Vedas are meant for knowing Kṛṣṇa. If one understands Kṛṣṇa from the Bhagavad-gītā and becomes situated in Kṛṣṇa consciousness, engaging himself in devotional service, he has reached the highest perfection of knowledge offered by the Vedic literature. Lord Caitanya Mahāprabhu made this process very easy: He asked people simply to chant Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare and to engage in the devotional service of the Lord and eat the remnants of foodstuff offered to the Deity. One who is directly engaged in all these devotional activities is to be understood as having studied all Vedic literature. He has come to the conclusion perfectly. Of course, for the ordinary persons who are not in Kṛṣṇa consciousness or who are not engaged in devotional service, what is to be done and what is not to be done must be decided by the injunctions of the Vedas. One should act accordingly, without argument. That is called following the principles of śāstra, or scripture. Śāstra is without the four principal defects that are visible in the conditioned soul: imperfect senses, the propensity for cheating, certainty of committing mistakes, and certainty of being illusioned. These four principal defects in conditioned life disqualify one from putting forth rules and regulations. Therefore, the rules and regulations as described in the śāstra – being above these defects – are accepted without alteration by all great saints, ācāryas and great souls.