Vedānta-sūtra says, athāto brahma jijñāsaḥ. Now here Arjuna is experienced, "You are the Supreme Brahman." So he has seen the Supreme Brahman. So you make Arjuna guru, Kṛṣṇa guru. Arjuna is representative of Kṛṣṇa, friend of Kṛṣṇa. So why do you go to a bogus guru? You must be cheated. Guru is essential. It is necessary. But take the real guru. But if you go to the bogus guru, you must be disappointed. For your treatment you need to go to a physician. That's all. When you are diseased you cannot say, "No, no, I don't want to . . ." It is necessary. But go to the real physician. Don't go to a cheater. He has no knowledge in the medical science, and he places himself as "I am physician, M.D," then you'll be cheated. The guru is necessary, that's a fact. But go to the real guru. Who is real guru? Real guru is Kṛṣṇa or one who has seen Kṛṣṇa, Arjuna. Take them. Then you'll be benefitted. And if you go to a bogus man who does not know Kṛṣṇa, who does not know what is Kṛṣṇa's instruction, then you must be cheated. So the answer is guru is absolute necessary. Tad-vijñānārthaṁ sa gurum evābhigacchet (MU 1.2.12). This is Vedic injunction, that one must go. But he must go to the real guru. And who is real guru? Who knows Kṛṣṇa. Take, for example, Arjuna, how he studied Kṛṣṇa. And he says, paraṁ brahma paraṁ dhāma (BG 10.12). Read the translation.
Puṣṭa Kṛṣṇa: "Arjuna said: You are the Supreme Brahman, the ultimate, the supreme abode and purifier, the Absolute Truth and the eternal divine person. You are the primal God, transcendental and original, and You are the unborn and all-pervading beauty. All the great sages such as Nārada, Asita, Devala and Vyāsa proclaim this of You, and now You Yourself are declaring it to me."
Prabhupāda: Arjuna is rectifying this because people may say, "Arjuna was Kṛṣṇa's friend, he is accepting Him as guru." No, Arjuna says, "Not only I, but other authorities, they also accept." So it is . . . everything is clear, that Kṛṣṇa is the Supreme Personality of Godhead and He should be accepted as guru or His representative should be accepted guru. Then it will be . . . So our this Kṛṣṇa consciousness movement is that we are presenting Kṛṣṇa as the supreme guru. You take instruction from Him and be benefited. But one who is carrying this message, he is also authorized. Just like one money order, it is coming through the post office, but an ordinary peon is handing over the money order. But he is representative of post office. Actually, the money order is being delivered by the post office, general post office. But it is coming through an ordinary peon. But because he is authorized to deliver you, he is also post office. He's as good as the post office. Just you have got a letter box, a small box, but if you put your letter there, your letter will surely go ten thousand miles away. Therefore, although it is a small box, you don't think it is small box; it is whole post office. Similarly, anyone who is carrying the message of Kṛṣṇa, don't think that he's ordinary man. If you imitate one box like that post box and put your letter, for thousand years it will lie down there, because it is not authorized. So if somebody says this small box, red box, is as good as the post office, one may say, "Huh, this is small box. How it can be as good as the post office?" But you see. You post your letter, it will go. Therefore, sākṣād-dharitvena samasta-śāstrair uktas tathā bhāvyata eva sadbhiḥ. Guru is directly the Supreme Personality of Godhead. Why? Because he is presenting the words of Supreme Personality of Godhead without any deterioration. Therefore he is so honored. So therefore the conclusion is guru is necessary, and guru is he who is representative of Kṛṣṇa. Otherwise he's bogus.