"Therefore we sing daily, yasya prasadad bhagavat-prasadah. By the mercy of guru, immediately we become, get the mercy of God. Yasya prasadad bhagavat-prasado yasyaprasadan na gatih kuto 'pi. Yasyaprasadat. If guru is not satisfied, then na gatih kuto 'pi. He has no gati. Gati means advancement. He has no advancement. Dhyayam stuvams tasya yasas tri-sandhyam. Therefore this Gurvastaka, samsara-davanala, we have to sing. So God's incarnation, here it is said, 'There are many incarnations.' So guru is also incarnation of God, mercy incarnation of God. Guru means that... God is within you, caitya-guru, the guru, or the spiritual master, within your heart. Isvarah sarva-bhutanam hrd-dese 'rjuna tisthati. So this Paramatma is also incarnation of God. And the same Paramatma, when He comes before you, being very much merciful upon you, to teach you from outside, that is guru. Therefore it is said, saksad-dharitvena samasta-sastraih. Guru is the mercy incarnation of God."
(Srila Prabhupada lecture, Los Angeles, October 1, 1972)