The important lesson learnt after 55 years in ISKCON.

In all my years in ISKCON my greatest lesson is-

"Blind belief in authority is the greatest enemy of truth."

So be very careful, do not accept everything blindly just because they are a Temple President, GBC, Sannyasi and especially a Guru. 

Research everything and realize a Guru is suppose to be a pure representative of Kṛṣṇa but never claim to be all knowing like Kṛṣṇa.

The individual jiva-souls (including pure devotees of Kṛṣṇa) do not know everything like Krsna does as Prabhupada explains here-

Srila Prabhupada - "God is also an individual person as you are individual person, I am individual person, but the difference between God and you and me is this, that you know your business, I know my business, but God knows everyone's business. That is the difference." (Lecture Melb, Australia April 3, 1972) 

So how do we know who is a bona fide spiritual master? For the serious spiritual candidate, that’s a very important question.

When an interviewer asked Srila Prabhupada that question, he replied- 

Srila Prabhupada - "Just like when you go to the market to purchase something, you test it whether it is genuine or not. Similarly, you have to test who is a genuine Guru and not just follow sentimentally and blindly." (Press Conference, Aug 5, 1971, London)

Srila Prabhupada - "A guru must prove his position not only by wearing a sacred thread but by advancing knowledge in spiritual life." (SB, Canto 5 Ch 10 text 16, Purport) 

Srila Prabhupada - "A guru must be a direct representative of Kṛṣṇa who distributes the instructions of Kṛṣṇa without any change." (SB, Canto 5 Ch 17 text 11, Purport)^