Srimad Bhagavatam Canto 10, Chapter 13, Text 54

SB 10.13.54

api hy upanisad-drsam
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
The visnu-murtis all had eternal, unlimited forms, full of knowledge and bliss and existing beyond the influence of time. Their great glory was not even to be touched by the jñanis engaged in studying the Upanisads.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
Mere sastra jñana, or knowledge in the Vedas, does not help anyone understand the Personality of Godhead. Only one who is favored or shown mercy by the Lord can understand Him. This is also explained in the Upanisads (Mundaka Upanisad 3.2.3):
nayam atma pravacanena labhyo
na medhasa na bahuna srutena
yam evaisa vrnute tena labhyas
tasyaisa atma vivrnute tanum svam
“The Supreme Lord is not obtained by expert explanations, by vast intelligence, or even by much hearing. He is obtained only by one whom He Himself chooses. To such a person, He manifests His own form.”
One description given of Brahman is satyam brahma, ananda-rupam: “Brahman is the Absolute Truth and complete ananda, or bliss.” The forms of Visnu, the Supreme Brahman, were one, but They were manifested differently. The followers of the Upanisads, however, cannot understand the varieties manifested by Brahman. This proves that Brahman and Paramatma can actually be understood only through devotion, as confirmed by the Lord Himself in Srimad-Bhagavatam: bhaktyaham ekaya grahyah (Bhag. 11.14.21). To establish that Brahman indeed has transcendental form, Srila Visvanatha Cakravarti Thakura gives various quotations from the sastras. In the Svetasvatara Upanisad (3.8), the Supreme is described as aditya-varnam tamasah parastat: “He whose self-manifest form is luminous like the sun and transcendental to the darkness of ignorance.” Ananda-matram ajaram puranam ekam santam bahudha drsyamanam: “The Supreme is blissful, with no tinge of unhappiness. Although He is the oldest, He never ages, and although one, He is experienced in different forms.” Sarve nityah sasvatas ca dehas tasya paratmanah: “All the forms of that Supreme Person are eternal.” (Maha-varaha Purana) The Supreme Person has a form, with hands and legs and other personal features, but His hands and legs are not material. Bhaktas know that the form of Krsna, or Brahman, is not at all material. Rather, Brahman has a transcendental form, and when one is absorbed in it, being fully developed in bhakti, one can understand Him (premañjana-cchurita-bhakti-vilocanena). The Mayavadis, however, cannot understand this transcendental form, for they think that it is material.
Transcendental forms of the Supreme Personality of Godhead in His person are so great that the impersonal followers of the Upanisads cannot reach the platform of knowledge to understand them. Particularly, the transcendental forms of the Lord are beyond the reach of the impersonalists, who can only understand, through the studies of the Upanisads, that the Absolute Truth is not matter and that the Absolute Truth is not materially restricted by limited potency.
Yet although Krsna cannot be seen through the Upanisads, in some places it is said that Krsna can in fact be known in this way. Aupanisadam purusam: “He is known by the Upanisads.” This means that when one is purified by Vedic knowledge, one is then allowed to enter into devotional understanding (mad-bhaktim labhate param).
tac chraddadhana munayo
pasyanty atmani catmanam
bhaktya sruta-grhitaya
“The seriously inquisitive student or sage, well equipped with knowledge and detachment, realizes that Absolute Truth by rendering devotional service in terms of what he has heard from the Vedanta-sruti.” (Bhag. 1.2.12) The word sruta-grhitaya refers to Vedanta knowledge, not sentimentality. Sruta-grhita is sound knowledge.
Lord Visnu, Brahma thus realized, is the reservoir of all truth, knowledge and bliss. He is the combination of these three transcendental features, and He is the object of worship for the followers of the Upanisads. Brahma realized that all the different forms of cows, boys and calves transformed into Visnu forms were not transformed by mysticism of the type that a yogi or demigod can display by specific powers invested in him. The cows, calves and boys transformed into visnu-murtis, or Visnu forms, were not displays of visnu-maya, or Visnu energy, but were Visnu Himself. The respective qualifications of Visnu and visnu-maya are just like those of fire and heat. In heat there is the qualification of fire, namely warmth; and yet heat is not fire. The manifestation of the Visnu forms of the boys, cows and calves was not like the heat, but rather like the firethey were all actually Visnu. Factually, the qualification of Visnu is full truth, full knowledge and full bliss. Another example may be given with material objects, which may be reflected in many, many forms. For example, the sun is reflected in many waterpots, but the reflections of the sun in many pots are not actually the sun. There is no actual heat and light from the sun in the pot, although it appears as the sun. But each and every one of the forms Krsna assumed was fully Visnu.
We should discuss Srimad-Bhagavatam daily as much as possible, and then everything will be clarified, for Bhagavatam is the essence of all Vedic literature (nigama-kalpa-taror galitam phalam). It was written by Vyasadeva (maha-muni-krte) when he was self-realized. Thus the more we read Srimad-Bhagavatam, the more its knowledge becomes clear. Each and every verse is transcendental.
Srimad Bhagavatam Canto 10, Chapter 13, Text 53
Srimad Bhagavatam Canto 10, Chapter 13, Text 55