(Srila Prabhupada and Kartamasi in Montreal, 1975)
"Kṛṣṇa is within everyone. Īśvaraḥ sarva-bhūtānāṁ hṛd-deśe 'rjuna tiṣṭhati (BG 18.61). So He is called caitya-guru, means guru within the heart. And when Kṛṣṇa understands... Because Kṛṣṇa knows everything, whatever we are willing, whatever we are doing. He is the Paramātmā; He's sitting within our hearts as a witness. Anumanta upadṛṣṭā. Upadṛṣṭā means simply observing what we are doing and what we are desiring. So as we desire, Kṛṣṇa 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 Kṛṣṇa sees that a living entity is very anxious to understand Him or to revive his Kṛṣṇa consciousness, then Kṛṣṇa gives him all opportunity, especially by manifesting Himself as the spiritual master. Antar bahiḥ. The spiritual master is therefore Kṛṣṇa's manifestation, Kṛṣṇa's mercy manifestation to help a person to develop his Kṛṣṇa 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 Kṛṣṇa consciousness."
(Srila Prabhupada Lecture, London, August 22, 1973)