From Bhakti: The Art of Eternal Love: Srila Prabhupada defines devotional service. What is it and how can one attain it?
The authorized descriptions of bhakti, or devotional service, following in the footsteps of previous ācāryas, can be summarized in the following statement by Śrīla Rūpa Gosvāmī: “First-class devotional service is known by one’s tendency to be fully engaged in Kṛṣṇa consciousness, serving the Lord favorably.” The purport is that one may also be in Kṛṣṇa consciousness unfavorably, but that cannot be counted as pure devotional service. Pure devotional service should be free from the desire for any material benefit or for sense gratification, as these desires are cultivated through fruitive activities and philosophical speculation. Generally, people are engaged in different activities to get some material profit, while most philosophers are engaged in proposing transcendental realization through volumes of word jugglery and speculation. Pure devotional service must always be free from such fruitive activities and philosophical speculations. One has to learn Kṛṣṇa consciousness, or pure devotional service, from the authorities by spontaneous loving service.
In the Nectar of Instruction, Rūpa Gosvāmī gives six unique features of pure devotional service:
1. Pure devotional service brings immediate relief from all kinds of material distress.
2. Pure devotional service is the beginning of all auspiciousness.
3. Pure devotional service automatically puts one in transcendental pleasure.
4. Pure devotional service is rarely achieved.
5. Those in pure devotional service deride even the conception of liberation.
6. Pure devotional service is the only means to attract Kṛṣṇa. Kṛṣṇa is all-attractive, but pure devotional service attracts even Him. This means that pure devotional service is even transcendentally stronger than Kṛṣṇa Himself, because it is Kṛṣṇa’s internal potency.