rājaguhyam - the king of secret, because what one seek is exactly what one is. One would never figure it out unless is told by some other source. It is like wanting to see our own eyes, we need a mirror. Word of śāstra is like a mirror to show us exactly what we want to be. We must see "I am brahman", till then brahman is the greatest secret.
pavitram idam utamam - it is the greatest purifier. Even though there are many purifying agents such as water and fire (gross) to disciplines and karma-yoga (subtle), there is nothing more purifying compared to ātmā-jñānam because it removes even the concept of purity and impurity (when we know everything here is only brahman). This purifier is the most exalted because it burns away countless births of the jīva along with their cause, punya and pāpa.
dharmyam - enjoined by / not apposed to śāstra. The derivation of the word dharma is "that which is hold", because the harmonious of the entire creation is supported/hold by dharma. Both dharma and self-knowledge are enjoined by śāstra therefore it is called dharmyam. And the knowledge of brahman is Īśvara himself, thus this knowledge is never separated or opposed to him or his order.
When all above descriptions are given for brahma-jñānam, one can feel frustrated with this pursue. Therefore Lord Krsna said susukham kartum - easy to accomplish, as long as one is well prepared and no obstruction, the knowledge will take place like how sight takes place when we employ our eyes. The words of śāstra take care of the job of revealing the nature of brahman directly as "I" - pratyaksāvagamam.
After introducing the subject matter, now Lord Krsna said that gaining this you will be released from bondage. Therefore, the first prerequisite for this knowledge is śraddhā, and how much you will miss if you don't have.
aśraddadhānāh purusā dharmasyāsya parantapa |
aprāpya mām nivartante mrtyusamsāravartmani ||9.3||
Arjuna, the scorcher of foes! Those people who have no faith in this self-knowledge, not gaining Me, they return to/remain in the road of samsāra, which is fraught with death.
Next, Lord Krsna reveals the relationship between brahman and this world.
mayā tatam-idam sarvam jagad avyaktamūrtinā |
matsthāni sarvabhūtāni na cāham tesvavasthitah ||9.4||
This entire world is pervaded by Me whose form cannot be objectified. All beings have their being in Me and I am not based in them.
mayā tatam-idam sarvam jagad - this entire world is pervaded by me, avyaktamūrtinā - whose form cannot be objectified. A form of a given thing is called mūrti. That form which cannot be objectified means it is beyond objectification, it doesn't mean non existence. If the nature of the brahman is having certain form which can be objectified, means it is limited by that certain form only. If it is said to be that certain form, means it is not the other form. This is called object limitation. In this case, the world which consists of many names and forms can't be his manifestation. Only the existence formless un-manifest can be the basis for the existence of other objects which have form. Thus this entire world is pervaded by brahma who is existence, consciousness, not limited by time, space, or any object.
matsthāni sarvabhūtāni - all beings have their being in me (brahman). Just like a gold chain which is made with gold, there is no part of the chain is not filled with gold, therefore the being (existence) of the chain is in gold alone. Without gold, there is no chain at all. Similarly all beings (the entire creation which consist of names and forms including my body-mind-sense-complex) have their being in brahman which is formless existence. The names and forms are incidental attributes (mithyā) which add on to the substantive existence brahman (satyam). Therefore the world including my body-mind-sense-complex comes and goes, but me ātmā which is the truth for all here is never effected.
There is a common notion that ātmā is staying and located in the heart (buddhi - intellect) of every being. If this is the case, brahman is endowed with location. To rule out that brahman has space wise limitation, Lord Krsna said na cāham tesvavasthitah - but I am not based in them. Formless brahman can't have any location just like space. Can we locate space in any location? Can we say that there is space inside the pot? Or actually the pot is in the space? It is correct that buddhi is the place where one recognises ātmā but it doesn't means ātmā is located there. Buddhi is a vrtti (thought wave) and every vrtti is nothing but manifestation of ātmā. A pot vrtti is awareness / consciousness with the name-form pot. Every vrtti depends upon ātmā like waves depend entirely upon water. Therefore brahman is not based in any being as another entity, but all being is nothing but brahman alone.