Here in the Bhagavad-gītā, the Supreme Personality of Godhead explaining Himself, Kṛṣṇa. So if you say, "How can I believe that Kṛṣṇa said? Somebody has written in the name of Kṛṣṇa that 'Kṛṣṇa said,' 'God said.' " No. This is called disciplic succession. You will see in this book, Bhagavad-gītā, Kṛṣṇa, what Kṛṣṇa said, and how Arjuna understood.
These things are described there. And the sādhu, saintly person, beginning from Vyāsadeva, Nārada, down to many ācāryas, Rāmānujācārya, Madhvācārya, Viṣṇu Svāmī, and latest, Lord Caitanya, in this way, they have accepted, "Yes. It is spoken by Kṛṣṇa." So this is the proof: if saintly persons have accepted. They have not denied. Authorities, they have accepted, "Yes." This is called sādhu.
And because sādhu, saintly persons, have accepted, therefore it is scripture. That is the test. Just like . . . it is common sense affair. If the lawyers accept some book, then it is to be understood that this is law book. You cannot say that "How can I accept this is law?" The evidence is the lawyers are accepting. Medical . . . if the medical practitioners accept, then that is authoritative medical book. Similarly, if saintly persons are accepting Bhagavad-gītā as scripture, you cannot deny it.