Ahaṁ sarvasya prabhavo mattaḥ sarvaṁ pravar. . ., iti matvā budhā bhāva-samanvitāḥ (BG 10.8). Budhā. Budhā means one who has understood thoroughly. So therefore he can become. . . One who has understood thoroughly Kṛṣṇa as the supreme controller, he is guru. He is guru. Otherwise one cannot be guru. Yei kṛṣṇa-tattva-vettā sei guru haya (CC Madhya 8.128). Who can (be) guru? Guru is not artificial thing, "Guru Mahārāja," "this Mahārāja. . .," no. One who has firmly understood that Kṛṣṇa is the original cause of everything, He is the Supreme Person—nobody can, I mean to say, remove him from that firm convictional position—then he is guru. Otherwise he is not guru. Guru is not so easy thing. Budhā bhāva-samanvitāḥ (BG 10.8). One who has understood Kṛṣṇa as the Supreme Personality of Godhead, the supreme controller, the supreme maintainer, the Supreme Person, everything, only one, vāsudevaḥ sarvam iti (BG 7.19)—that is guru. But otherwise, he is not guru, one who has not understood Kṛṣṇa.
Therefore Caitanya Mahāprabhu confirms, yei kṛṣṇa-tattva-vettā sei guru haya (CC Madhya 8.128). One who has understood Kṛṣṇa 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 the Caitanya Mahāprabhu's formula. Therefore, first of all try to understand Kṛṣṇa. 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 Mahārāja. No. That is not guru. Sa mahātmā. . . Vāsudevaḥ sarvam iti sa mahātmā sudurlabhaḥ (BG 7.19).
So their theory, this atheistic theory, is aparaspara-sambhūtam. It has taken by mechanical arrangement. Mechanical. . . Kim anyat kāma-haitukam (BG 16.8). Kāma. Just like kāma—a man, a woman becomes lusty, and there is sex, and there is production. So they say like that. The production is like that. There is no other plan. But there is a big plan. That they do not know. The plan is: yadā yadā hi dharmasya glānir bhavati bhārata, tadātmānaṁ sṛjāmy aham (BG 4.7).