[ādi-guru means the teacher of all teachers, or original teacher].
From Śrī Guru Darśanam, pp. 177 – 178
Question: Previously we have heard that Balarāma and Nityānanda are the ādiguru, and also that the guru is a manifestation of Śrimatī Rādhārāni. Is that true?
Answer (by Maharajji): These statements are not based on śāstra. It is all right if the people who are propagating this have their own understanding, but they too are supposed to be following śāstra.
Suppose you want a servant to do some favorable personal service. You will train him yourself, because you want the service to be done in a specific way. It is not that you appoint somebody else to train the person. You do it yourself to make sure you get exactly what you want. If some other person trains them, then the service will be done according to that person’s mood. It is Krṣṇa, whom we are supposed to serve, and it is Krṣṇa Himself who takes care of training his people. Krṣṇa himself promises to maintain this process. The guru is Krṣṇa himself, and not anybody else.
Balarāma, Nityānanda or Śrimatī Rādhārāni have different purposes in Krsna’s pastimes, although there is no difference between Balarāma and Nityānanda. They assist Bhagavān in his pastimes, but they are not guru themselves. Nowhere in the śāstra does it say that Balarāma is ādiguru. Krṣṇa himself says that He manifests Himself as guru (SB 11.17.27)
“ācāryam mām vijānīyān nāvamanyeta karhicit |
na martya-buddhyāsūyeta sarva-deva-mayo guruḥ ||
Know the ācārya to be my manifestation, never disrespect him, and do not find fault in him, thinking him to be a mortal being. Know the guru as embodiment of all the devas. ”
Question: We should deal with the guru as Krṣṇa. How shall I understand this?
Answer: This means being respectful, not disobeying, not disrespecting or criticizing the guru. When it is said that the guru is to be treated like God, it means that just as you do not criticize God, you also do not find fault with the guru. You take his instructions as real and authoritative.
Question: Should one also learn from the guru how to approach Krṣṇa?
Answer: Yes. This is what guru means. Guru means teacher, and this is why we are here.