manusyanam sahasresu
kascid yatati siddhaye
yatatam api siddhanam
kascin mam vetti tattvatah

manusyanam–of men; sahasresu–out of many thousands; kascit–someone; yatati–endeavors; siddhaye–for perfection; yatatam–of those so endeavoring; api–indeed; siddhanam–of those who have achieved perfection; kascit–someone; mam–Me; vetti–does know; tattvatah–in fact.

TRANSLATION
Out of many thousands among men, one may endeavor for perfection, and of those who have achieved perfection, hardly one knows Me in truth.

PURPORT

There are various grades of men, and out of many thousands, one may be sufficiently interested in transcendental realization to try to know what is the self, what is the body, and what is the Absolute Truth. Generally mankind is simply engaged in the animal propensities, namely eating, sleeping, defending and mating, and hardly anyone is interested in transcendental knowledge. The first six chapters of the Gita are meant for those who are interested in transcendental knowledge, in understanding the self, the Superself and the process of realization by jnana-yoga, dhyana-yoga and discrimination of the self from matter. However, Krsna can be known only by persons who are in Krsna consciousness. Other transcendentalists may achieve impersonal Brahman realization, for this is easier than understanding Krsna. Krsna is the Supreme Person, but at the same time He is beyond the knowledge of Brahman and Paramatma. The yogis and jnanis are confused in their attempts to understand Krsna. Although the greatest of the impersonalists, Sripada Sankaracarya, has admitted in his Gita commentary that Krsna is the Supreme Personality of Godhead, his followers do not accept Krsna as such, for it is very difficult to know Krsna, even though one has transcendental realization of impersonal Brahman.
Krsna is the Supreme Personality of Godhead, the cause of all causes, the primeval Lord Govinda. Isvarah paramah krsnah sac-cid-ananda-vigrahah/ anadir adir govindah sarva-karana-karanam. It is very difficult for the nondevotees to know Him. Although nondevotees declare that the path of bhakti, or devotional service, is very easy, they cannot practice it. If the path of bhakti is so easy, as the nondevotee class of men proclaim, then why do they take up the difficult path? Actually the path of bhakti is not easy. The so-called path of bhakti practiced by unauthorized persons without knowledge of bhakti may be easy, but when it is practiced factually according to the rules and regulations, the speculative scholars and philosophers fall away from the path. Srila Rupa Gosvami writes in his Bhakti-rasamrta-sindhu (1.2.101):

sruti smrti-puranadi-
pancaratra-vidhim vina
aikantiki harer bhaktir
utpatayaiva kalpate

“Devotional service of the Lord that ignores the authorized Vedic literatures like the Upanisads, puranas and Narada-pancaratra is simply an unnecessary disturbance in society.”
It is not possible for the Brahman-realized impersonalist or the Paramatma-realized yogi to understand Krsna the Supreme Personality of Godhead as the son of mother Yasoda or the charioteer of Arjuna. Even the great demigods are sometimes confused about Krsna (muhyanti yat surayah). Mam tu veda na kascana: “No one knows Me as I am,” the Lord says. And if one does know Him, then sa mahatma su-durlabhah. “Such a great soul is very rare.” Therefore unless one practices devotional service to the Lord, one cannot know Krsna as He is (tattvatah), even though one is a great scholar or philosopher. Only the pure devotees can know something of the inconceivable transcendental qualities in Krsna, in the cause of all causes, in His omnipotence and opulence, and in His wealth, fame, strength, beauty, knowledge and renunciation, because Krsna is benevolently inclined to His devotees. He is the last word in Brahman realization, and the devotees alone can realize Him as He is. Therefore it is said:

atah sri-krsna-namadi
na bhaved grahyam indriyaih
sevonmukhe hi jihvadau
svayam eva sphuraty adah

“No one can understand Krsna as He is by the blunt material senses. But He reveals Himself to the devotees, being pleased with them for their transcendental loving service unto Him.” (Bhakti-rasamrta-sindhu 1.2.234)