SB 4.29.3

yo ’vijñātāhṛtas tasya
 puruṣasya sakheśvaraḥ
yan na vijñāyate pumbhir
 nāmabhir vā kriyā-guṇaiḥ
Word for word: 
yaḥ — he who; avijñāta — unknown; āhṛtaḥ — described; tasya — of him; puruṣasya — of the living entity; sakhā — the eternal friend; īśvaraḥ — the master; yat — because; na — never; vijñāyate — is understood; pumbhiḥ — by the living entities; nāmabhiḥ — by names; — or; kriyā-guṇaiḥ — by activities or qualities.
Translation: 
The person I have described as unknown is the Supreme Personality of Godhead, the master and eternal friend of the living entity. Since the living entities cannot realize the Supreme Personality of Godhead by material names, activities or qualities, He remains everlastingly unknown to the conditioned soul.
Purport: 

Because the Supreme Personality of Godhead is unknown to the conditioned soul, He is sometimes described in Vedic literatures as nirākāra, avijñāta or avāṅ-mānasa-gocara. Actually it is a fact that the Supreme Personality of Godhead cannot be perceived by material senses as far as His form, name, quality, pastimes or paraphernalia are concerned. However, when one is spiritually advanced, one can understand the name, form, qualities, pastimes and paraphernalia of the Supreme Lord. This is confirmed in Bhagavad-gītā (18.55). Bhaktyā mām abhijānāti yāvān yaś cāsmi tattvataḥ: one can understand in truth the Supreme Personality of Godhead only when one is engaged in devotional service. Ordinary persons engaged in pious and impious activities cannot understand the form, name and activities of the Lord. The devotee, however, can know the Personality of Godhead in many respects. He can understand that Kṛṣṇa is the Supreme Personality of Godhead, that His address is Goloka Vṛndāvana and that His activities are all spiritual. Because the Lord’s form and activities cannot be understood by materialistic people, He is described by the śāstras as nirākāra, that is, one whose form cannot be ascertained by a materialistic person. This does not mean that the Supreme Personality of Godhead has no form; it means that it is not understood by the karmīs, or fruitive actors. His form is described in Brahma-saṁhitā as sac-cid-ānanda-vigraha. As confirmed by the Padma Purāṇa:

ataḥ śrī-kṛṣṇa-nāmādi
 na bhaved grāhyam indriyaiḥ
sevonmukhe hi jihvādau
 svayam eva sphuraty adaḥ

“No one can understand Kṛṣṇa as He is by utilizing the blunt material senses. However, the Lord reveals Himself to His devotees, being pleased with them because of their transcendental loving service rendered unto Him.”

Since the name, form, qualities and activities of the Supreme Personality of Godhead, Kṛṣṇa, cannot be understood by the material senses, He is also called adhokṣaja, meaning “beyond sense perception.” When the senses are purified by devotional activity, the devotee understands everything about the Lord by the Lord’s grace. In this verse the words pumbhir nāmabhir vā kriyā-guṇaiḥ are especially significant because God, Kṛṣṇa, the Supreme Personality of Godhead, has many names, activities and qualities, although none of them are material. Despite the fact that all these names, activities and pastimes are mentioned in the śāstras and understood by the devotees, the karmīs (fruitive laborers) cannot understand them. Nor can the jñānīs (mental speculators) understand them. Although there are thousands of names of Lord Viṣṇu, the karmīs and jñānīs intermingle the names of the Supreme Godhead with the names of demigods and human beings. Because they cannot understand the actual name of the Supreme Personality of Godhead, they take it for granted that any name can be accepted. They believe that since the Absolute Truth is impersonal, they can call Him by any name. Otherwise, they maintain, He has no name. This is not a fact. Here it is clearly stated: nāmabhir vā kriyā-guṇaiḥ. The Lord has specific names such as Rāma, Kṛṣṇa, Govinda, Nārāyaṇa, Viṣṇu and Adhokṣaja. There are indeed many names, but the conditioned soul cannot understand them.