When the path of omkara-upāsana is very exalted with the result of brahma-loka where moksa is gained in sequence, one might conclude that, this is the path for moksa. Therefore Lord Krsna reveals here, the self-knowledge which giving the result of freedom from life of becoming is immediate, not later and there.
idam tu te guhyatamam pravaksyāmyanasūyave |
jñānam vijñanasahitam yajjñātvā moksyase' śubhāt ||9.1||
Now, I will clearly explain to you (who are) without calumny, this most secret knowledge together with immediate knowledge, knowing which you will be released from all that is inauspicious.
Lord Krsna said he will explain this brahman in detailed. The word "tu - however" is used to indicate what is going to talk about here is different from the topic about two paths in the eight chapter. This is a special path where no traveling is required, it is only by owning up this knowledge, one is free now and here.
For such a special means, who is qualified for this knowledge? anasūyave (anasūyu) - one who can accept good qualities or accomplishment in others. This is only one of the values one should cultivate as a mature person ready to receive ātmā-jñānam. As it is said that Bhagavad Gītā is not just teaching brahma-vidyā (knowledge of brahman), but also yoga-sāstra (the means preparing one to be qualified for brahma-vidyā). Thus in the entire Gītā, Lord Krsna keeps mentioning about the qualification of the seeker of moksa.
This knowledge is guhyatamam - the greatest secret, because even though it is taught, it is not understood. No matter how many times it being taught, rarely it is understood by anyone, just like what verse 2.29 talked about. Even if it is understood, there are certain things that inhibit the assimilation of the knowledge. One needs certain degree of viveka (discrimination capacity) and vairagya (grown out of attachment) which don't come by itself, one has to work on them. The preparedness for this knowledge is the greatest thing one can accomplish in life. Another reason for this knowledge being the most secret, is that it is not something that can be gained by our known means of knowledge like perception or inference, or by our experience. Because everything we can objectify is not brahman. The witness itself is brahman, therefore whatever is witnessed is not brahman. Thus there is no way of knowing "I am brahman", unless you have another pramānam - means of knowledge, which is śāstra. Since it is not available for any means of knowledge we have, it is said as greatest secret. Also when thing is precious or rare, it is kept secret. So saying it is the most secret also means it is the most precious.
jñānam vijñanasahitam - this knowledge of brahman is also immediate knowledge as yourself. The teacher doesn't simply state that there is brahman, but also to prove that you are brahman. It will bring you an immediate appreciation of this knowledge as yourself.
What will I gain from this knowledge? yajjñātvā moksyase' śubhāt - having which you will be free from inauspicious/undesirable such as sorrow, sense of smallness, bondage, etc - samsāra.
To draw the attention of Arjuna, Lord Krsna said.....
rājavidyā rājaguhyam pavitram idam uttamam |
pratyaksāvagamam dharmyam susukham kartum avyayam ||9.2||
This is the king of all knowledge, the king of secrets, the greatest purifier, directly appreciated, not opposed to dharma, easy to accomplish and imperishable.
This knowledge is rājavidyā - the king among all disciplines of knowledge. Śankara-ācārya gives the meaning of the word rāja drawn from its root rāj which means to shine. What is the thing which illumines all and doesn't need any other thing else to illumine itself? It is only ātmā, the only self-effulgent (self-known) and without which nothing is known. Therefore this knowledge of ātmā is called the king of all knowledge because all other forms of knowledge depend upon this illumining factor called ātmā to be revealed. And also every knowledge about any subject matter is never total. But by having knowledge of ātmā, one knows the nature of the entire universe. Therefore it is called as the king of knowledge.