Srimad Bhagavatam Canto 10, Chapter 02, Text 36

SB 10.2.36

na nama-rupe guna-janma-karmabhir
 nirupitavye tava tasya saksinah
mano-vacobhyam anumeya-vartmano
 deva kriyayam pratiyanty athapi hi
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
O Lord, Your transcendental name and form are not ascertained by those who merely speculate on the path of imagination. Your name, form and attributes can be ascertained only through devotional service.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
As stated in the Padma Purana:
atah sri-krsna-namadi
 na bhaved grahyam indriyaih
sevonmukhe hi jihvadau
 svayam eva sphuraty adah
“One cannot understand the transcendental nature of the name, form, quality and pastimes of Sri Krsna through one’s materially contaminated senses. Only when one becomes spiritually saturated by transcendental service to the Lord are the transcendental name, form, quality and pastimes of the Lord revealed to him.” Since Krsna and His transcendental name, form and activities are all of a transcendental nature, ordinary persons or those who are only slightly advanced cannot understand them. Even big scholars who are nondevotees think that Krsna is fictitious. Yet although so-called scholars and commentators do not believe that Krsna was factually a historical person whose presence on the Battlefield of Kuruksetra is recorded in the history of Mahabharata, they feel compelled to write commentaries on Bhagavad-gita and other historical records. Sevonmukhe hi jihvadau svayam eva sphuraty adah: Krsna’s transcendental name, form, attributes and activities can be revealed only when one engages in His service in full consciousness. This confirms Krsna’s own words in Bhagavad-gita (18.55):
bhaktya mam abhijanati
 yavan yas casmi tattvatah
tato mam tattvato jñatva
 visate tad-anantaram
“One can understand the Supreme Personality of Godhead as He is only by devotional service. And when one is in full consciousness of the Supreme Lord by such devotion, he can enter into the kingdom of God.” Only by sevonmukha, by engaging oneself in the Lord’s service, can one realize the name, form and qualities of the Supreme Personality of Godhead.
“O Lord,” the demigods say, “the impersonalists, who are nondevotees, cannot understand that Your name is identical with Your form.” Since the Lord is absolute, there is no difference between His name and His actual form. In the material world there is a difference between form and name. The mango fruit is different from the name of the mango. One cannot taste the mango fruit simply by chanting, “Mango, mango, mango.” But the devotee who knows that there is no difference between the name and the form of the Lord chants Hare Krsna, Hare Krsna, Krsna Krsna, Hare Hare/ Hare Rama, Hare Rama, Rama Rama, Hare Hare, and realizes that he is always in Krsna’s company.
For persons who are not very advanced in absolute knowledge of the Supreme, Lord Krsna exhibits His transcendental pastimes. They can simply think of the pastimes of the Lord and get the full benefit. Since there is no difference between the transcendental name and form of the Lord, there is no difference between the transcendental pastimes and the form of the Lord. For those who are less intelligent (like women, laborers or the mercantile class), the great sage Vyasadeva wrote Mahabharata. In the Mahabharata, Krsna is present in His different activities. Mahabharata is history, and simply by studying, hearing, and memorizing the transcendental activities of Krsna, the less intelligent can also gradually rise to the standard of pure devotees.
The pure devotees, who are always absorbed in the thought of the transcendental lotus feet of Krsna and who are always engaged in devotional service in full Krsna consciousness, are never to be considered to be in the material world. Srila Rupa Gosvami has explained that those who are always engaged in Krsna consciousness by body, mind and activities are to be considered liberated even within this body. This is also confirmed in the Bhagavad-gita: those who are engaged in the devotional service of the Lord have already transcended the material position.
Krsna appears in order to give a chance to both the devotees and the nondevotees for realization of the ultimate goal of life. The devotees get the direct chance to see Him and worship Him. Those who are not on that platform get the chance to become acquainted with His activities and thus become elevated to the same position.
The Brahma-samhita (5.38) says:
 santah sadaiva hrdayesu vilokayanti
yam syamasundaram acintya-guna-svarupam
 govindam adi-purusam tam aham bhajami
Although Krsna’s transcendental form is presented as black, devotees who are in love with the Supreme Personality of Godhead appreciate the Lord as Syamasundara, having a very beautiful blackish form. The Lord’s form is so beautiful that the Brahma-samhita (5.30) also states:
venum kvanantam aravinda-dalayataksam
 barhavatamsam asitambuda-sundarangam
 govindam adi-purusam tam aham bhajami
“I worship Govinda, the primeval Lord, who plays on His transcendental flute. His eyes are like lotus flowers, He is decorated with peacock plumes, and His bodily color resembles the color of a fresh black cloud, although His bodily features are more beautiful than millions of Cupids.” This beauty of the Supreme Lord can be seen by devotees who are in love with Him, devotees whose eyes are anointed with love of Godhead (premañjana-cchurita-bhakti-vilocanena).
The Lord is also known as Giridhari or Girivara-dhari. Because Krsna, for the sake of His devotees, lifted Govardhana Hill, the devotees appreciate the Lord’s inconceivable strength; but nondevotees, in spite of directly perceiving the Lord’s inconceivable strength and power, regard the Lord’s activities as fictitious. This is the difference between a devotee and a nondevotee. Nondevotees cannot give any nomenclature for the Supreme Personality of Godhead, yet the Lord is known as Syamasundara and Giridhari. Similarly, the Lord is known as Devaki-nandana and Yasoda-nandana because He accepted the role of son for mother Devaki and mother Yasoda, and He is known as Gopala because He enjoyed the sport of maintaining the cows and calves. Therefore, although He has no mundane name, He is addressed by devotees as Devaki-nandana, Yasoda-nandana, Gopala and Syamasundara. These are all transcendental names that only devotees can appreciate and nondevotees cannot.
The history of Krsna the person has been openly seen by everyone, yet only those who are in love with the Supreme Personality of Godhead can appreciate this history, whereas nondevotees, who have not developed their loving qualities, think that the activities, form and attributes of the Supreme Personality of Godhead are fictitious. Therefore this verse explains, na nama-rupe guna janma-karmabhir nirupitavye tava tasya saksinah. In this connection, Srila Visvanatha Cakravarti Thakura has given the example that persons suffering from jaundice cannot taste the sweetness of sugar candy, although everyone knows that sugar candy is sweet. Similarly, because of the material disease, nondevotees cannot understand the transcendental name, form, attributes and activities of the Supreme Personality of Godhead, although they actually see the Lord’s activities, either through authority or through history. The Puranas are old, authentic histories, but nondevotees cannot understand them, especially Srimad-Bhagavatam, which is the essence of Vedic knowledge. Nondevotees cannot understand even the preliminary study of transcendental knowledge, Bhagavad-gita. They simply speculate and present commentaries with absurd distortions. In conclusion, unless one elevates himself to the transcendental platform by practicing bhakti-yoga, one cannot understand the Supreme Personality of Godhead or His name, form, attributes or activities. But if by chance, by the association of devotees, one can actually understand the Lord and His features, one immediately becomes a liberated person. As the Lord says in Bhagavad-gita (4.9):
janma karma ca me divyam
 evam yo vetti tattvatah
tyaktva deham punar janma
 naiti mam eti so ’rjuna
“One who knows the transcendental nature of My appearance and activities does not, upon leaving the body, take his birth again in this material world, but attains My eternal abode, O Arjuna.”
Srila Rupa Gosvami has therefore said that by affection and love for the Supreme Personality of Godhead, devotees can express their mind to Him with their words. Others, however, cannot do this, as confirmed in Bhagavad-gita (bhaktya mam abhijanati yavan yas casmi tattvatah).
Srimad Bhagavatam Canto 10, Chapter 02, Text 35
Srimad Bhagavatam Canto 10, Chapter 02, Text 37