No one can understand why this Supreme God comes to earth as an ordinary human being and executes such wonderful, uncommon activities. One should know, then, that scholarship is not the qualification necessary to understand Kṛṣṇa. Even the demigods and the great sages have tried to understand Kṛṣṇa by their mental speculation, and they have failed to do so. In the Śrīmad-Bhāgavatam also it is clearly said that even the great demigods are not able to understand the Supreme Personality of Godhead. They can speculate to the limits of their imperfect senses and can reach the opposite conclusion of impersonalism, of something not manifested by the three qualities of material nature, or they can imagine something by mental speculation, but it is not possible to understand Kṛṣṇa by such foolish speculation.
(Bhagavad-Gita-----10:2-----purport).

When a person knows the goal of life but is addicted to the fruits of activities, he is acting in karma-yoga. When he knows that the goal is Kṛṣṇa but he takes pleasure in mental speculations to understand Kṛṣṇa, he is acting in jñāna-yoga. And when he knows the goal and seeks Kṛṣṇa completely in Kṛṣṇa consciousness and devotional service, he is acting in bhakti-yoga, or buddhi-yoga, which is the complete yoga. This complete yoga is the highest perfectional stage of life.
(Bhagavad-Gita-----10:10-----purport).

Due to the contamination of material association, through many, many millions of births, one's heart is always covered with the dust of materialism, but when one engages in devotional service and constantly chants Hare Kṛṣṇa, the dust quickly clears, and one is elevated to the platform of pure knowledge. The ultimate goal, Viṣṇu, can be attained only by this chant and by devotional service, and not by mental speculation or argument. The pure devotee does not have to worry about the material necessities of life; he need not be anxious, because when he removes the darkness from his heart, everything is provided automatically by the Supreme Lord, who is pleased by the loving devotional service of the devotee. This is the essence of the teachings of Bhagavad-gītā. By studying Bhagavad-gītā, one can become a soul completely surrendered to the Supreme Lord and engage himself in pure devotional service. As the Lord takes charge, one becomes completely free from all kinds of materialistic endeavors.
(Bhagavad-Gita-----10:11-----purport).

The Supreme Lord, Kṛṣṇa, can be known by persons who are in a relationship with Him through the discharge of devotional service, like Arjuna and his followers. Persons of demonic or atheistic mentality cannot know Kṛṣṇa. Mental speculation that leads one away from the Supreme Lord is a serious sin, and one who does not know Kṛṣṇa should not try to comment on Bhagavad-gītā. Bhagavad-gītā is the statement of Kṛṣṇa, and since it is the science of Kṛṣṇa, it should be understood from Kṛṣṇa as Arjuna understood it. It should not be received from atheistic persons.
(Bhagavad-Gita-----10:15-----purport).

The word yogeśvara is also very significant here because the Lord has inconceivable power. If He likes, He can reveal Himself by His grace, although He is unlimited. Therefore Arjuna pleads for the inconceivable grace of Kṛṣṇa. He does not give Kṛṣṇa orders. Kṛṣṇa is not obliged to reveal Himself unless one surrenders fully in Kṛṣṇa consciousness and engages in devotional service. Thus it is not possible for persons who depend on the strength of their mental speculations to see Kṛṣṇa.
(Bhagavad-Gita-----11:4-----purport).

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.
(Bhagavad-Gita-----11:54-----purport).

My dear Arjuna, he who engages in My pure devotional service, free from the contaminations of fruitive activities and mental speculation, he who works for Me, who makes Me the supreme goal of his life, and who is friendly to every living being—he certainly comes to Me.
(Bhagavad-Gita-----11:55-----purport).

In other words, knowledge concerning the spirit soul beyond this body is called knowledge in the mode of goodness, knowledge producing many theories and doctrines by dint of mundane logic and mental speculation is the product of the mode of passion, and knowledge concerned only with keeping the body comfortable is said to be in the mode of ignorance.
(Bhagavad-Gita-----18:22-----purport).

The Supreme Personality of Godhead, Kṛṣṇa, and His plenary portions cannot be understood by mental speculation nor by the nondevotees. If anyone wants to understand the Supreme Personality of Godhead, he has to take to pure devotional service under the guidance of a pure devotee. Otherwise, the truth of the Supreme Personality of Godhead will always be hidden. As already stated in Bhagavad-gītā (7.25), nāhaṁ prakāśaḥ sarvasya: He is not revealed to everyone. No one can understand God simply by erudite scholarship or mental speculation. Only one who is actually engaged in Kṛṣṇa consciousness and devotional service can understand what Kṛṣṇa is. University degrees are not helpful.
(Bhagavad-Gita-----18:55-----purport).

No one can properly describe the transcendental nature of the Absolute Truth. Therefore it is said that He is beyond the expression of mind and speech. And yet there are some men, with a poor fund of knowledge, who desire to understand the Absolute Truth by imperfectmental speculation and faulty description of His activities. To the layman His activities, appearance and disappearance, His names, His forms, His paraphernalia, His personalities and all things in relation with Him are mysterious.
(Srimad Bhagavatam-----1:3:37-----purport).

The fruitive workers have practically no information of the Absolute Truth, and the mental speculators, after being frustrated in fruitive activities, turn their faces towards the Absolute Truth and try to know Him by mental speculation. And for all these men, the Absolute Truth is a mystery, as the jugglery of the magician is a mystery to children. Being deceived by the jugglery of the Supreme Being, the nondevotees, who may be very dexterous in fruitive work and mental speculation, are always in ignorance.
(Srimad Bhagavatam-----1:3:37-----purport).