There are many expansion of the Supreme Personality of Godhead: Nārāyaṇa, Adhokṣaja . . . there are many, innumerable planets in the spiritual world, and all of them are emanation from the Kṛṣṇa planet. And the Kṛṣṇa planet, the supreme Deity is Kṛṣṇa.
This description we have got in several Vedic literatures—Brahma-saṁhitā and Vedānta-sūtra, Bhāgavata, in Purāṇas, in Upaniṣad. Everywhere these descriptions are there. Those who are scholarly student, they'll get information, and the whole thing is summarized in Bhagavad-gītā. Therefore Bhagavad-gītā is known also as Gītopaniṣad. At the end of every chapter you'll find these things are written, gītāsu or upaniṣatsu. It is one of the Upaniṣads. You have read Upaniṣad. The Bhagavad-gītā is also one of the Upaniṣads, and therefore Bhagavad-gītā is known as Gītopaniṣad.
So if nobody knows the Supreme Personality of Godhead, then how He can be known? If He is not known to the demigods, if He is (not) known to the great sages, then how He can be known? Yes. He can be known. He can be known in this way. When the Supreme Lord comes before you and He reveals Himself before you, then you can know.
You cannot know by your imperfect senses. Our senses are imperfect. We have . . . several times we have described in this meeting that our senses cannot realize the Supreme Truth. Ataḥ śrī-kṛṣṇa-nāmadi na bhaved grāhyam indriyaiḥ (CC Madhya 17.136). Our senses are so imperfect that simply by expanding the sense power artificially, speculating, we cannot realize. These are described in different way in different scriptures.
So He can be known how? That is also described—sevonmukhe hi jihvādau svayam eva sphuraty adaḥ: when you become in a submissive attitude and you chant. Jihvādau. Jihvādau means the realization begins from the tongue. God realization begins from the tongue. Therefore the tongue's function is to eat and to vibrate sound. So these two processes, vibration of Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare / Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare, and eating kṛṣṇa-prasāda. You cannot allow your tongue to taste anything and everything.