In the seventh chapter, Lord Krsna was unfolding to Arjuna knowledge (wisdom) with realization (real knowledge of manifest Divinity). But in between, Arjuna at the beginning of the eighth chapter, raised seven questions. So Lord Krsna, by answering the first six questions, in brief, answered, in detail the seventh question—"How are you to be known at the time of death by the self-controlled?" Now, the Lord starts the same topic of Knowledge (Jnana), with Realization (Vijnana), in the ninth chapter.
idam tu te guhyatamarh pravaksyamyanasayave
jnanam vijnanasahitarit yajjnatva moksyase'subhat
- The Blessed Lord said
To you who is free from the carping spirit I shall now unfold the most mysterious knowledge (Jnana), along with realization (Vijnana) by knowing which, you will be released from evil i.e., the evil of worldly birth and death. 1
Idam to te guhyatammh pravaksyamyanasuyave:—Lord Krsna, has used the term 'Idam' (this), to highlight the topic, which he wants to unfold. To state its merit, He has used the superlative form, of the adjective—'Guhyatamam' (most profound). It means, that this knowledge combined with realization, is most profound. It has been called, a 'sovereign secret', in the second verse, and 'the greatest secret', in the sixty-fourth verse of the eighteenth chapter.
Here, in this chapter, fast He used the expression 'Guhyatarnam' (the most profound), and later (in 9/34) used 'Manmanabhava (fix your mind on Me), while, in the eighteenth chapter, first He used the expression 'Sarvaguhyatamam' (the most secret of all), and then (in 18/65) said 'Manmanabhava' (fix your mind on Me). It means, that the same topic has been explained here, as well as, there.
This most profound secret cannot be disclosed to all, because the Lord has described His own merit, in it. One who possesses a critical spirit, in the least, can take the contrary meaning, that Lord Krsna, is boasting of His own merits. The Lord wants to say to Arjuna, that he is devoid of a carping spirit. So He, will unfold the most profound knowledge, along with realization to him viz., He will explain the Tattva' and also the methods for its realization.
Secondly, He means to say, that every human being is eligible to take refuge in Him. Even the vilest sinner, belonging to any caste, creed, country and colour, deserves to seek refuge, in Him. So, by taking refuge in Him, he can attain Him. Hence He wants to disclose this secret.