"Yada yada hi dharmasya.. glanir bhavati bharata
abhyutthanam adharmasya.. tadatmanam srjamy aham
paritranaya sadhunam.. vinashaya cha duskrtam
dharma-samsthapanarthaya.. sambhavami yuge yuge.."
(from 'Bhagavad Gita)
Translation : "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 order to deliver the pious and to annihilate the miscreants, as well as to reestablish the principles of religion, I advent Myself millennium after millennium."