"Therefore having a bona fide spiritual master and serving him and pleasing him and getting his mercy is essential. Otherwise there can be no advancement in Krishna consciousness. And unless the spiritual master is a pure devotee of Krishna then he has no potency to give you Krishna. He is simply a cheating rascal.
"So in fact, above all the rules and regulations and offenses I have mentioned the most important thing, the essential thing, which is required if you want to come to the stage of purely chanting the Hare Krishna mantra is you must have a bona fide spiritual master who is a pure devotee of Krishna. Without having a bona fide spiritual master you can chant Hare Krishna forever but you will not be able to advance because Krishna does not reveal Himself in this way. He only reveals Himself to those devotees who surrender to and serve and please His pure devotees."
(Srila Prabhupada letter to his London disciples, July, 1969)