Prabhupada Letters :: 1969.

"Even Lord Krishna, Lord Caitanya, and what to speak of other acaryas, all of them accepted a bona fide Spiritual Master. Aroha means to try to understand God by one's own effort, and avaroha means to understand God by disciplic succession."

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."