(Srila Prabhupada and disciples, Toronto, 1976)
"Now, due to this modern age, Kali-yuga, it is very difficult to find out a qualified brāhmaṇa. So therefore it is very difficult also to find out a qualified spiritual master. So Caitanya Mahāprabhu, He has recommended that,
kibā vipra, kibā nyāsī, śūdra kene naya
yei kṛṣṇa-tattva-vettā sei 'guru' haya
(CC Madhya 8.128)
'Never mind whether he's a brāhmaṇa or a śūdra or a sannyāsī or a householder. Never mind what he is. That doesn't matter. If he knows the science of Kṛṣṇa, then he is the bona fide spiritual master.' He is the bona fide spiritual master. So here this Bhagavad-gītā is the science of Kṛṣṇa. Every one of you, if you study Bhagavad-gītā very scrutinizingly, with all your arguments, with all your sense, with all your philosophical knowledge... Because it is said here, tad viddhi praṇipātena paripraśnena (BG 4.34).
Not that you have to submit yourself blindly. Although your spiritual master may be self-realized and experienced in the Absolute Truth, still, you have to question. You have to understand from him all critical points by your intelligent questions. That is allowed. So it doesn't matter. If anyone is able to answer about the science of Kṛṣṇa, he is spiritual master. He is spiritual master. It doesn't matter where he's born or what he is, whether he's a brāhmaṇa, or a śūdra, or an American or an Indian or whatever he is. Never mind.
Just like... It is practical. When you go to a doctor, medical practitioner, you do not ask, 'Well, Doctor, are you American or Indian? Are you brāhmaṇa? Are you Jew? Are you Christian?' No. Oh, he has got the qualification of a medical man, so you surrender, 'Doctor, treat me. I am suffering.' So there is no question. So Caitanya Mahāprabhu has given liberal that 'Anyone who knows the spiritual science, or the Kṛṣṇa science...' Kṛṣṇa science is spiritual science. Because Kṛṣṇa is the ultimate goal of spiritual science."
(Srila Prabhupada Lecture, New York, August 17, 1966)