SB 2.7.19

tubhyaṁ ca nārada bhṛśaṁ bhagavān vivṛddha-
 bhāvena sādhu parituṣṭa uvāca yogam
jñānaṁ ca bhāgavatam ātma-satattva-dīpaṁ
 yad vāsudeva-śaraṇā vidur añjasaiva
Synonyms: 
tubhyam — unto you; ca — also; nārada — O Nārada; bhṛśam — very nicely; bhagavān — the Personality of Godhead; vivṛddha — developed; bhāvena — by transcendental love; sādhu — your goodness; parituṣṭaḥ — being satisfied; uvāca — described; yogam — service; jñānam — knowledge; ca — also; bhāgavatam — the science of God and His devotional service; ātma — the self; sa-tattva — with all details; dīpam — just like the light in the darkness; yat — that which; vāsudeva-śaraṇāḥ — those who are souls surrendered unto Lord Vāsudeva; viduḥ — know them; añjasā — perfectly well; eva — as it is.
Translation: 
O Nārada, you were taught about the science of God and His transcendental loving service by the Personality of Godhead in His incarnation of Haṁsāvatāra. He was very much pleased with you, due to your intense proportion of devotional service. He also explained unto you, lucidly, the full science of devotional service, which is especially understandable by persons who are souls surrendered unto Lord Vāsudeva, the Personality of Godhead.
Purport: 

The devotee and devotional service are two correlative terms. Unless one is inclined to be a devotee of the Lord, he cannot enter into the intricacies of devotional service. Lord Śrī Kṛṣṇa wanted to explain the Bhagavad-gītā, which is the science of devotional service, unto Śrī Arjuna because Arjuna was not only His friend but a great devotee as well. The whole process is that all living entities, being constitutionally parts and parcels of the supreme living being, the Absolute Personality of Godhead, have proportionately minute independence of action also. So the preliminary qualification for entering into the devotional service of the Lord is that one become a willing cooperator, and as such, one should voluntarily cooperate with persons who are already engaged in the transcendental devotional service of the Lord. By cooperating with such persons, the prospective candidate will gradually learn the techniques of devotional service, and with the progress of such learning one becomes proportionately free from the contamination of material association. Such a purificatory process will establish the prospective candidate in firm faith and gradually elevate him to the stage of transcendental taste for such devotional service. Thus he acquires a genuine attachment for the devotional service of the Lord, and his conviction carries him on to the point of ecstasy, just prior to the stage of transcendental love.

Such knowledge of devotional service may be divided into two sections, namely preliminary knowledge of the nature of devotional service and the secondary knowledge of its execution. Bhāgavatam is in relation with the Personality of Godhead, His beauty, fame, opulence, dignity, attraction and transcendental qualities which attract one towards Him for exchanges of love and affection. There is a natural affinity of the living entity for the loving service of the Lord. This affinity becomes artificially covered by the influence of material association, and Śrīmad-Bhāgavatam helps one very genuinely remove that artificial covering. Therefore it is particularly mentioned herein that Śrīmad-Bhāgavatam acts like the lamp of transcendental knowledge. These two sections of transcendental knowledge in devotional service become revealed to a person who is a soul surrendered unto Vāsudeva; as it is said in the Bhagavad-gītā (7.19), such a great soul, fully surrendered unto the lotus feet of Vāsudeva, is very, very rare.