Śrī Caitanya-caritāmṛta - Ādi-līlā 3.22-23

"“ ‘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."

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"“ ‘To deliver the pious and to annihilate the miscreants, as well as to reestablish the principles of religion, I Myself appear, millennium after millennium.’"

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Śrī caitanya-caritāmṛta Ādi 3.22 & 23