174
Śrīmad-Bhāgavatam
[Canto 8, Ch. 14]
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.
TRANSLATION
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
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):
kaler doṣa-nidhe rājann 
    asti hy eko mahān guṇaḥ 
    kīrtanād eva kṛṣṇasya 
    mukta-saṅgaḥ paraṁ vrajet
The entire Kali-yuga is full of faults. It is like an unlimited ocean of faults. But the Kṛṣṇa consciousness movement is very authorized.