Prabhupada Letters :: 1967.

"The secret of success in the matter of Krishna Consciousness is devotion to the Lord and the Spiritual Master. So continue your present attitude and you will be successful even I am not physically present before you."

Prabhupada Letters :: 1970.

"Yes, that is the way of understanding the secret of Krsna Consciousness. That is the Vedic way. Unflinching faith in Krsna and similarly in the bona fide Spiritual Master. That is the best qualification for spreading the movement of Lord Caitanya."

Prabhupada Letters :: 1968.

"That is the secret of Krishna Consciousness. The Spiritual Master must be bona fide representative of Krishna, and the disciple must agree to abide by His orders. This is the simple method of spiritual advancement."

Prabhupada Letters :: 1969.

"The secret of success in Krishna consciousness is to render service to Krishna to the best of our capacity। There is no other alternative to this. The more we render service therefore, the more we are able to advance."

Prabhupada Letters :: 1972.

"The more one renders service, the more he becomes enthusiastic. This is the secret of spiritual life, when one can work all day long and not be tired. That is real spiritual life."