SB 2.9.31

śrī-bhagavān uvāca
jñānaṁ parama-guhyaṁ me
 yad vijñāna-samanvitam
sarahasyaṁ tad-aṅgaṁ ca
 gṛhāṇa gaditaṁ mayā
Synonyms: 
śrī-bhagavān uvāca — the Personality of Godhead said; jñānam — knowledge acquired; parama — extremely; guhyam — confidential; me — of Me; yat — which is; vijñāna — realization; samanvitam — coordinated; sa-rahasyam — with devotional service; tat — of that; aṅgam ca — necessary paraphernalia; gṛhāṇa — just try to take up; gaditam — explained; mayā — by Me.
Translation: 
The Personality of Godhead said: Knowledge about Me as described in the scriptures is very confidential, and it has to be realized in conjunction with devotional service. The necessary paraphernalia for that process is being explained by Me. You may take it up carefully.
Purport: 

Lord Brahmā is the topmost devotee of the Lord within the universe, and therefore the Personality of Godhead replied to his four principal inquiries in four important statements, which are known as the original Bhāgavatam in four verses. These were Brahmā’s questions: (1) What are the forms of the Lord both in matter and in transcendence? (2) How are the different energies of the Lord working? (3) How does the Lord play with His different energies? (4) How may Brahmā be instructed to discharge the duty entrusted to Him? The prelude to the answers is this verse under discussion, wherein the Lord informs Brahmā that knowledge of Him, the Supreme Absolute Truth, as it is stated in the revealed scriptures, is very subtle and cannot be understood unless one is self-realized by the grace of the Lord. The Lord says that Brahmā may take the answers as He explains them. This means that transcendental knowledge of the absolute Supreme Being can be known if it is made known by the Lord Himself. By the mental speculation of the greatest mundane thinkers, the Absolute Truth cannot be understood. The mental speculators can reach up to the standard of impersonal Brahman realization, but, factually, complete knowledge of transcendence is beyond the knowledge of impersonal Brahman. Thus it is called the supreme confidential wisdom. Out of many liberated souls, someone may be qualified to know the Personality of Godhead. In the Bhagavad-gītā it is also said by the Lord Himself that out of many hundreds of thousands of people, one may try for perfection in human life, and out of many liberated souls, one may know Him as He is. Therefore, the knowledge of the Personality of Godhead may be attained by devotional service only. Rahasyam means devotional service. Lord Kṛṣṇa instructed Arjuna in the Bhagavad-gītā because He found Arjuna to be a devotee and friend. Without such qualifications, one cannot enter into the mystery of the Bhagavad-gītā. Therefore, one cannot understand the Personality of Godhead unless one becomes a devotee and discharges devotional service. This mystery is love of Godhead. Therein lies the main qualification for knowing the mystery of the Personality of Godhead. And to attain the stage of transcendental love of Godhead, regulative principles of devotional service must be followed. The regulative principles are called vidhi-bhakti, or the devotional service of the Lord, and they can be practiced by a neophyte with his present senses. Such regulative principles are mainly based on hearing and chanting of the glories of the Lord. And such hearing and chanting of the glories of the Lord can be made possible in the association of devotees only. Lord Caitanya therefore recommended five main principles for attaining perfection in the devotional service of the Lord. The first is association with devotees (hearing); second is chanting the glories of the Lord; third, hearing Śrīmad-Bhāgavatam from the pure devotee; fourth, residing in a holy place connected with the Lord; and fifth, worshiping the Deity of the Lord with devotion. Such rules and regulations are parts of devotional service. So, as requested by Lord Brahmā, the Personality of Godhead will explain all about the four questions put forward by Brahmā, and others also which are parts and parcels of the same questions.