SB 8.14.4
catur-yugante kalena
grastañ chruti-ganan yatha
tapasa rsayo ’pasyan
yato dharmah sanatanah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
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.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
In this verse, the words dharmah and sanatanah are very important. Sanatana 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 Treta-yuga, however, these principles are somewhat neglected, and only three fourths of the religious duties continue. In Dvapara-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 sanatana-dharma is very prominent. The duty of saintly persons, therefore, is to take up seriously the cause of sanatana-dharma and try to reestablish it for the benefit of the entire human society. The Krsna consciousness movement has been started according to this principle. As stated in Srimad-Bhagavatam (12.3.51):
kaler dosa-nidhe rajan
asti hy eko mahan gunah
kirtanad eva krsnasya
mukta-sangah param vrajet
The entire Kali-yuga is full of faults. It is like an unlimited ocean of faults. But the Krsna consciousness movement is very authorized. Therefore, following in the footsteps of Sri Caitanya Mahaprabhu, who five hundred years ago inaugurated the movement of sankirtana, krsna-kirtana, 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 sanatana-dharma, the eternal occupational duties of humanity. The eternal occupational duty of the human being is to serve Krsna. Jivera ‘svarupa’ haya-krsnera ‘nitya-dasa.’ This is the purport of sanatana-dharma. Sanatana means nitya, or “eternal,” and krsna-dasa means “servant of Krsna.” The eternal occupational duty of the human being is to serve Krsna. This is the sum and substance of the Krsna consciousness movement.