BG Chap 9 - Confidential knowledge taught by Kṛṣṇa
Kṛṣṇa stated in the seventh and eighth chapters that He is easily attained by pure bhakti. In this chapter the majestic aspect of the Lord as the object of worship suitable for His servants, and the excellence of pure devotee are described. Kṛṣṇa speaks of His supreme powers which cause astonishment and incite bhakti, as well as the great excellence of bhakti directed to Himself. The Gītā is the essence of all scriptures, and the essence of the Gītā is the middle six chapters. The middle of those six chapters, the ninth and tenth chapters, is the further essence of the six chapters.
Kṛṣṇa says that He would impart the most confidential knowledge, bhakti, along with realization of Him to Arjuna because Arjuna is never envious of the Lord. Knowing this one would be free from all sufferings. The knowledge of the ātmā distinct from the body, useful for liberation taught in the second, third, and subsequent chapters, is confidential. The knowledge taught in the seventh and eighth chapters useful for attaining the Lord is more confidential because that knowledge is actually knowledge of Bhagavān. The knowledge which would be taught in this chapter is knowledge about kevala or śuddha bhakti, which is most confidential. The term confidential indicates Kṛṣṇa is teaching advanced knowledge, and the more advanced the knowledge is, the less number of persons would understand it. Confidential also means the knowledge shared among an inner group of people, who are the devotees in this case. Thus even though Kṛṣṇa is teaching this knowledge openly, still only a few can understand it, and it is in this sense the knowledge is said to be confidential.