Bhagavad Gita Verse 20, Chapter 15

iti guhyatamam shaastramidamuktam mayaanagha |
etadbuddhvaa buddhimaansyaatkritakrityashcha bhaarata || 20 ||
 
Thus, this foremost secret has been taught to you by me, O sinless one. Having known this, one becomes wise and accomplishes all his duties, O Bhaarata.
 
iti : thus
guhyatamam : foremost secret
shaastram : science
idam : this
uktam : taught
mayaa : by me
anagha : O sinless one
etat : this
buddhvaa : having known
buddhimaan : wise
syaat : becomes
kritakrityaha : one who has accomplished all duties
cha : and
bhaarata : O Bhaarata
 
When you ask the question - can you stop working right now and retire, you get a couple of answers. Some people say that they have still so many desires, so many plans to fulfill, that’s why they cannot retire. Other people say that they still have so much to learn from the world, so much knowledge to acquire. Shri Krishna concludes this chapter by asserting that one who has truly understood the teaching of this chapter has accomplished whatever anyone can accomplish in this world, plus he has also known whatever can be known in this world.
 
Why does he say that whatever has to be known has been covered in this chapter? The highest knowledge to be known in this world is the understanding of three topics. What is the nature of the individual soul (who am I), what is this world and where did it come from, and what is beyond this world (is there a God). Any text that conclusively answers these three questions is termed a shaastra, a science. The fifteenth chapter of the Gita does so, and hence it is worthy of being termed a shaastra.
 
The method used to reveal Purushottama, the pure eternal essence, is to gradually move from the tangible to the intangible, from the visible to the subtle, from the visible universe to the invisible Prakriti to Purushottama who is beyond both. This method is known as Arundhati nyaaya, the technique of revealing the location of the star known as Arundhati. Here the teacher first points to a tree, then to one of its branches, then to one of its leaves, and then to the star that is right next to the tree. Without doing this step by step revelation, it would not have been possible to reveal the position of the star.
 
So then, the teaching of this chapter is called the foremost secret. It is secret because such knowledge is not accessible to any of these sense organs. It has to be revealed through a teacher who has had direct experience of the eternal essence. Furthermore, it has to be taught to a student who is straightforward and without sin like Arjuna. Shri Shankaraachaarya goes so far as to say that this chapter summarizes the teachings of all of the Vedic scriptures.
 
om tatsatiti shreematbhagavatgitasupanishadsu brahmavidyaayaam yogashaastre shreekrishnaarjunsamvade purushottamyogo naama panchadashodhyaayaha || 15 ||

Bhagavad Gita Verse 1, Chapter 9

Shree Bhagavaan uvaacha:
idam tu te guhyatamam pravakshyaamyanasooyave |
jnyaanam vijnyaanasahitam yajnyaatvaa mokshayaseshubhaat || 1 ||
 
Shree Bhagavaan said:
However, to you who is without fault, I will teach this extremely secret knowledge along with wisdom, having known which, you will be liberated from the inauspicious.
 
idam : this
tu : however
te : to you
guhyatamam : extremely secret
pravakshyaami : I will teach
anasooyave : faultless
jnyaanam : knowledge
vijnyaana : wisdom
sahitam : along with
yat : which
jnyaatvaa : having known
mokshayase : you will be liberated
ashubhaat : inauspicious
 
Shri Krishna uses this chapter to progress the theme of the seventh chapter, which was the infinite nature of Ishvara and the finite nature of maaya. He begins the chapter by asserting that the knowledge of Ishvara’s infinite nature will result in liberation. He addresses Arjuna as “anasooya” which means without fault, doubt or prejudice, indicating that those who have begun the process of purification of their minds through karma yoga and devoted meditation will understand this knowledge completely.
 
First, let us understand the result of knowledge that Shri Krishna is glorifying in this shloka. It is going to give us freedom or liberation from the inauspicious. The word inauspicious in this shloka refers to samsaara or the endless cycle of creation and dissolution that all of us are trapped in.
 
Next, let us look at what makes this knowledge unique. Shri Krishna says that he is going to reveal not just "jnyaana" or knowledge, but also “vijnyana” or wisdom that we can internalize in our lives. Also, unlike other knowledge that requires action to give a result, this knowledge gives us the result of liberation all by itself. As an example, if we come to know that a mirage in a desert is false, we don’t have to do anything further.
 
What makes this knowledge even more special is that it is extremely subtle. It is hard for someone to figure out on their own. We need a competent teacher like Shri Krishna to reveal this knowledge to us, as well as a competent student who is ready and unprejudiced. Shri Krishna, urging us to learn this knowledge in the right way through a teacher, calls it secret knowledge. It also shows the level of trust that Arjuna built with Shri Krishna in order to qualify for this knowledge.
 
Shri Krishna further glorifies this special knowledge in the next shloka.
 
Footnotes
1. Four things comprise ignorance or incorrect knowledge about our self. Doership, enjoyership, the notion of birth, death and rebirth, and the notion that the "I" in me has a different identity than the universe; all this is ignorance or avidya.
2. This ignorance is driven away by knowledge, which is the true nature of our self.