"Krishna is within everyone. Isvarah sarva-bhutanam hrd-dese 'rjuna tisthati [Bg. 18.61]. So He is called caitya-guru, means guru within the heart. And when Krishna understands... Because Krishna knows everything, whatever we are willing, whatever we are doing. He is the Paramatma; He's sitting within our hearts as witness. Anumanta upadrsta. Upadrsta means simply observing what we are doing, and what we are desiring. So as we desire, Krishna is very kind, He gives us facility. We have many times explained. Therefore, we have got varieties of bodies to enjoy this material world. So when Krishna sees that a living entity is very anxious to understand Him or to revive his Krishna consciousness, then Krishna gives him all opportunity, especially by manifesting Himself as the spiritual master. Antar bahih. The spiritual master is therefore Krishna's manifestation -- Krishna's mercy manifestation to help a person to develop his Krishna consciousness. From within He's helping. Therefore, He's called caitya-guru, and from without, He sends His representative to help how to become advanced in Krishna consciousness."
(Srila Prabhupada lecture, London, August 22, 1973)