One who has understood thoroughly Krsna as the supreme controller, he is guru. He is guru. Otherwise one cannot be guru. Yei krsna-tattva-vetta sei guru haya [Cc. Madhya 8.128]. Who can [be] guru? Guru is not artificial thing, “Guru Maharaja,” “this Maharaja…,” no. One who has firmly understood that Krsna is the original cause of everything, He is the Supreme Person — nobody can remove him from that firm convictional position — then he is guru. Otherwise he is not guru. Guru is not so easy thing. Budha bhava-samanvitah [Bg. 10.8]. One who has understood Krsna as the Supreme Personality of Godhead, the supreme controller, the supreme maintainer, the Supreme Person, everything, only one, vasudevah sarvam iti [Bg. 7.19] — that is guru. But otherwise, he is not guru, one who has not understood Krsna.
Therefore Caitanya Mahaprabhu confirms, yei krsna-tattva-vetta sei guru haya [Cc. Madhya 8.128]. One who has understood Krsna as the supreme controller, the Supreme Person, the supreme maintainer, and everything the supreme, ultimate, then he can become guru. Otherwise, don’t try to become guru. Impersonalists, half-understood, partial-understood, he cannot become guru. This is Caitanya Mahaprabhu’s formula. Therefore first of all try to understand Krsna. You will understand in such a way that you can refute all others’ argument, all others’ opposition. There are so many opposing elements. Then you are guru. Otherwise you cannot become a guru. Guru is not so teeny thing or trifle thing that everyone becomes Guru Maharaja. No. That is not guru.
From Srila Prabhupada’s lecture on Bhagavad-gita 16.8 — January 28, 1975, Tokyo