What does it mean to say that Vishnu worships those who worship him?
This is a recurring theme throughout the scriptures. Krishna says in the Uddhava Gita (Chapter 9):
Neither Brahma, nor Shiva, nor Balarama, nor Lakshmi, nor My own form is so dear to Me as you. [addressing Uddhava, who Krishna says is the supreme devotee of the Lord]
With a view to purifying Myself by the dust of his feet, I always follow the sage who cares for nothing, is calm, bears enmity to none, and is even-minded.
O Uddhava, neither Yoga, nor knowledge, nor piety, nor study, nor austerity, nor renunciation captivates Me so much as a heightened devotion to Me.
And in the Gita:
VI. 30. He who sees Me everywhere and sees everything in Me, to him I am never lost, nor is he ever lost to Me.
VI. 31. He who having been established in oneness, worships Me dwelling in all beings--that yogi, in whatever way he leads his life, lives in Me.
VII. 17. ...For supremely dear am I to the man of wisdom, and he is dear to Me.
IX. 29. I am the same to all beings; to Me there is none hateful or dear. But those who worship Me with devotion--they are in Me, and I too am in them.
The Lord cannot resist true Bhakti, true love which asks for nothing in return. He is drawn to true love as iron is drawn to a magnet. Thus, as it says in the Uddhava Gita, he follows a true sage in order to get the dust of his feet.
In the Shakta tradition, the Divine Mother is sometimes shown with dust on Her - this is the dust of the feet of Her true devotees, as She will lay on the ground so that they may walk over Her so that She can collect the dust of true love.
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