Srimad Bhagavatam Canto 09, Chapter 24, Text 56

SB 9.24.56

yada yada hi dharmasya
 ksayo vrddhis ca papmanah
tada tu bhagavan isa
 atmanam srjate harih
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
Whenever the principles of religion deteriorate and the principles of irreligion increase, the supreme controller, the Personality of Godhead Sri Hari, appears by His own will.
 
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
The principles by which an incarnation of the Supreme Personality of Godhead descends upon earth are explained in this verse. The same principles are also explained in Bhagavad-gita (4.7) by the Lord Himself:
 
yada yada hi dharmasya
 glanir bhavati bharata
abhyutthanam adharmasya
 tadatmanam srjamy aham
 
“Whenever and wherever there is a decline in religious practice, O descendant of Bharata, and a predominant rise of irreligion — at that time I descend Myself.”
 
In the present age, the Supreme Personality of Godhead has appeared as Sri Caitanya Mahaprabhu to inaugurate the Hare Krsna movement. At the present time, in Kali-yuga, people are extremely sinful and bad (manda). They have no idea of spiritual life and are misusing the benefits of the human form to live like cats and dogs. Under these circumstances Sri Caitanya Mahaprabhu has inaugurated the Hare Krsna movement, which is not different from Krsna, the Supreme Personality of Godhead. If one associates with this movement, he directly associates with the Supreme Personality of Godhead. People should take advantage of the chanting of the Hare Krsna mantra and thus gain relief from all the problems created in this Age of Kali.
Srimad Bhagavatam Canto 09, Chapter 24, Text 53-55
Srimad Bhagavatam Canto 09, Chapter 24, Text 57