After hearing Bhagavad-gītā from the Supreme Personality of Godhead, Arjuna accepted Kṛṣṇa as paraṁ brahma, the Supreme Brahman. Every living being is Brahman, but the supreme living being, or the Supreme Personality of Godhead, is the Supreme Brahman. Paraṁ dhāma means that He is the supreme rest or abode of everything; pavitram means that He is pure, untainted by material contamination; puruṣam means that He is the supreme enjoyer; śāśvatam, original; divyam, transcendental; ādi-devam, the Supreme Personality of Godhead; ajam, the unborn; and vibhum, the greatest.
Now one may think that because Kṛṣṇa was the friend of Arjuna, Arjuna was telling Him all this by way of flattery, but Arjuna, just to drive out this kind of doubt from the minds of the readers of Bhagavad-gītā, substantiates these praises in the next verse when he says that Kṛṣṇa is accepted as the Supreme Personality of Godhead not only by himself but by authorities like Nārada, Asita, Devala and Vyāsadeva. These are great personalities who distribute the Vedic knowledge as it is accepted by all ācāryas. Therefore Arjuna tells Kṛṣṇa that he accepts whatever He says to be completely perfect. Sarvam etad ṛtaṁ manye: "I accept everything You say to be true." Arjuna also says that the personality of the Lord is very difficult to understand and that He cannot be known even by the great demigods. This means that the Lord cannot even be known by personalities greater than human beings. So how can a human being understand Lord Śrī Kṛṣṇa without becoming His devotee?
Therefore Bhagavad-gītā should be taken up in a spirit of devotion. One should not think that he is equal to Kṛṣṇa, nor should he think that Kṛṣṇa is an ordinary personality or even a very great personality. Lord Śrī Kṛṣṇa is the Supreme Personality of Godhead. So according to the statements of Bhagavad-gītā or the statements of Arjuna, the person who is trying to understand the Bhagavad-gītā, we should at least theoretically accept Śrī Kṛṣṇa as the Supreme Personality of Godhead, and with that submissive spirit we can understand the Bhagavad-gītā. Unless one reads the Bhagavad-gītā in a submissive spirit, it is very difficult to understand Bhagavad-gītā, because it is a great mystery.