Hayagrīva: God intervenes in the world through the individual acting according to scripture.
Prabhupāda: Yes. These is a word, sādhu śāstra guru vākya cittete koriyā aikya. We can approach God by understanding a saintly person, by studying the Vedic scriptures, and explained directly by the bona fide spiritual master. So sādhu means saintly person, and śāstra means scriptures, and guru means spiritual master—and that they should be corroborated. A sādhu is he who talks in terms of scripture. Similarly, guru is he who talks in terms of scripture. Guru cannot manufacture words which is not in the scriptures. And that is not scripture which does not tally with the words of guru and sādhu. So these three items should be corroborated, and then we can understand who is guru, who is sādhu, and who is, what is scripture. Then we take instruction from them, and we can perfectly make progress towards understanding of the Supreme Personality of Godhead.
Hayagrīva: He says, "The only thing which enters Him, God, who is all-majestic, is obedience."
Prabhupāda: Yes.
Hayagrīva: "It is so easy to see that one to whom everything is equally important and equally insignificant can only be interested in one thing: obedience."
Prabhupāda: Yes. That is wanted, and Kṛṣṇa, or God, demands that. Full obedience. Sarva-dharmān parityajya mām ekaṁ śaraṇaṁ vraja (BG 18.66). That is the qualification. Tad viddhi praṇipātena (BG 4.34). So original obedience is to the Supreme Personality of Godhead, similarly obedience to the spiritual master is representative of God. So anyone who carries out the order of God, he can become bona fide guru, because he is not manufacturing anything. He is simply presenting what God is speaking, or the śāstra is speaking. God, when He comes as incarnation, He does not speak anything which is not in the scripture. That, just like in the Bhagavad-gītā, Kṛṣṇa gives reference to the Brahma-sūtra, Vedānta-sūtra. He is God. Whatever He is speaking, that is final, that's, that's a fact. Still, He is giving honor to the Vedānta-sūtra. Brahma-sūtra-padaiś caiva hetumadbhir viniścitaiḥ (BG 13.5). He is giving reference to the Brahma-sūtra because spiritual knowledge is asserted there with logic and philosophy. So we cannot accept anyone as incarnation of God if He speaks nonsense, not corroborating with the standard scripture.