grastāñ chruti-gaṇān yathā
tapasā ṛṣayo ’paśyan
yato dharmaḥ sanātanaḥ
catuḥ-yuga-ante—at the end of every four yugas (Satya, Dvāpara, Tretā and Kali); kālena—in due course of time; grastān—lost; śruti-gaṇān—the Vedic instruction; yathā—as; tapasā—by austerity; ṛṣayaḥ—great saintly persons; apaśyan—by seeing misuse; yataḥ—wherefrom; dharmaḥ—occupational duties; sanātanaḥ—eternal.
At the end of every four yugas, the great saintly persons, upon seeing that the eternal occupational duties of mankind have been misused, reestablish the principles of religion.
In this verse, the words dharmaḥ and sanātanaḥ are very important. Sanātana means “eternal,” and dharma means “occupational duties.” From Satya-yuga to Kali-yuga, the principles of religion and occupational duty gradually deteriorate. In Satya-yuga, the religious principles are observed in full, without deviation. In Tretā-yuga, however, these principles are somewhat neglected, and only three fourths of the religious duties continue. In Dvāpara-yuga only half of the religious principles continue, and in Kali-yuga only one fourth of the religious principles, which gradually disappear. At the end of Kali-yuga, the principles of religion, or the occupational duties of humanity, are almost lost. Indeed, in this Kali-yuga we have passed through only five thousand years, yet the decline of sanātana-dharma is very prominent. The duty of saintly persons, therefore, is to take up seriously the cause of sanātana-dharma and try to reestablish it for the benefit of the entire human society. The Kṛṣṇa consciousness movement has been started according to this principle. As stated in Śrīmad-Bhāgavatam (12.3.51):
The entire Kali-yuga is full of faults. It is like an unlimited ocean of faults. But the Kṛṣṇa consciousness movement is very authorized. Therefore, following in the footsteps of Śrī Caitanya Mahāprabhu, who five hundred years ago inaugurated the movement of saṅkīrtana, kṛṣṇa-kīrtana, we are trying to introduce this movement, according to superior orders, all over the world. Now, if the inaugurators of this movement strictly follow the regulative principles and spread this movement for the benefit of all human society, they will certainly usher in a new way of life by reestablishing sanātana-dharma, the eternal occupational duties of humanity. The eternal occupational duty of the human being is to serve Kṛṣṇa. Jīvera ‘svarūpa’ haya—kṛṣṇera ‘nitya-dāsa.’ [Cc. Madhya 20.108]. This is the purport of sanātana-dharma. Sanātana means nitya, or “eternal,” and kṛṣṇa-dāsa means “servant of Kṛṣṇa.” The eternal occupational duty of the human being is to serve Kṛṣṇa. This is the sum and substance of the Kṛṣṇa consciousness movement.
Link to this page: https://prabhupadabooks.com/sb/8/14/4