BG Chap 9 - Characteristics of bhakti
(a) King of all knowledge and king of all secrets
This knowledge is the king of all types of knowledge (rāja-vidyā) such as śāṇḍilya-vidyā, vaiṣvānara-vidyā, and dahara-vidyā, which are all mentioned in the Upaniṣads. Among the principal philosophers in India are Gautama, Kaṇāda, Kapila, Yājñavalkya, Śāṇḍilya and Vaiśvānara. And finally there is Vyāsadeva, the author of the Vedānta-sūtra. So there is no dearth of knowledge in the field of philosophy or transcendental knowledge. But the Lord says that this is the king of all such knowledge. Since knowledge here refers to bhakti, this is the highest bhakti exceeding all others. This is the king of secret matters (rāja-guhyaṁ) concerning the real nature of the jīvātmā and other items. Bhakti of any type is very secret, and of all types of bhakti, this is the king, the most secret.
Rāja vidyā and rāja guhyaṁ also means knowledge belonging to the kings and the secret belonging to the kings. This knowledge belongs to persons like kings who are generous and compassionate, and who thus consider even heaven insignificant. It is not the knowledge of those with selfish hearts worshipping devatās with desire for quick gain of sons or wealth. Just as kings do not keep their treasures secret, but attentively keep their personal mantras secret, though they are generous, so the devotees, though generous, keep this knowledge secret with great care, though they do not keep other types of knowledge secret.
(b) Most purifying
This knowledge is supremely pure because it destroys all sins including the subtle body, where the karmas are stored. With the disappearance of all karmas, the subtle body dissolves and one attains liberation. Sins in the form of aprārabdha (whose effects will manifest in future lifetimes), kūṭa (dormant), bīja (seed like state) (both stages are intermediate stages before which the effects manifest), and prārabdha (effects manifest this lifetime as enjoyment and suffering) are gradually destroyed for a person engaged in worship of Viṣṇu. As a needle pierces one leaf at a time in a pile of leaves, so bhakti will destroy karmas one at a time. Thus this knowledge is most purifying.