Kṛṣṇa can be understood only by the process of undivided devotional service. He explicitly explains this in this verse so that unauthorized commentators, who try to understand Bhagavad-gītā by the speculative process, will know that they are simply wasting their time. No one can understand Kṛṣṇa or how He came from parents in a four-handed form and at once changed Himself into a two-handed form. These things are very difficult to understand by study of the Vedas or by philosophical speculation. Therefore it is clearly stated here that no one can see Him or enter into understanding of these matters. Those who, however, are very experienced students of Vedic literature can learn about Him from the Vedic literature in so many ways. There are so many rules and regulations, and if one at all wants to understand Kṛṣṇa, he must follow the regulative principles described in the authoritative literature. One can perform penance in accordance with those principles. For example, to undergo serious penances one may observe fasting on Janmāṣṭamī, the day on which Kṛṣṇa appeared, and on the two days of Ekādaśī (the eleventh day after the new moon and the eleventh day after the full moon). As far as charity is concerned, it is plain that charity should be given to the devotees of Kṛṣṇa who are engaged in His devotional service to spread the Kṛṣṇa philosophy, or Kṛṣṇa consciousness, throughout the world. Kṛṣṇa consciousness is a benediction to humanity. Lord Caitanya was appreciated by Rūpa Gosvāmī as the most munificent man of charity because love of Kṛṣṇa, which is very difficult to achieve, was distributed freely by Him. So if one gives some amount of his money to persons involved in distributing Kṛṣṇa consciousness, that charity, given to spread Kṛṣṇa consciousness, is the greatest charity in the world. And if one worships as prescribed in the temple (in the temples in India there is always some statue, usually of Viṣṇu or Kṛṣṇa), that is a chance to progress by offering worship and respect to the Supreme Personality of Godhead. For the beginners in devotional service to the Lord, temple worship is essential, and this is confirmed in the Vedic literature (Śvetāśvatara Upaniṣad 6.23):
- yasya deve parā bhaktir
- yathā deve tathā gurau
- tasyaite kathitā hy arthāḥ
- prakāśante mahātmanaḥ
- (ŚU 6.23)
One who has unflinching devotion for the Supreme Lord and is directed by the spiritual master, in whom he has similar unflinching faith, can see the Supreme Personality of Godhead by revelation. One cannot understand Kṛṣṇa by mental speculation. For one who does not take personal training under the guidance of a bona fide spiritual master, it is impossible to even begin to understand Kṛṣṇa. The word tu is specifically used here to indicate that no other process can be used, can be recommended, or can be successful in understanding Kṛṣṇa.
The personal forms of Kṛṣṇa, the two-handed form and the four-handed, are completely different from the temporary universal form shown to Arjuna. The four-handed form of Nārāyaṇa and the two-handed form of Kṛṣṇa are eternal and transcendental, whereas the universal form exhibited to Arjuna is temporary. The very word sudurdarśam, meaning "difficult to see," suggests that no one had seen that universal form. It also suggests that amongst the devotees there was no necessity of showing it. That form was exhibited by Kṛṣṇa at the request of Arjuna so that in the future, when one represents himself as an incarnation of God, people can ask to see his universal form.