Chapter 2,Verse 29

ascharyavat pashyati kashchit enam
ascharyavad vadathi tathaiva chanyaha
ascharyavachchainam anya srunothi
sruthvapyenam veda na chaiva kashchit

Krishna now comes back from his suppositional argument that we are the mortal body,which he had taken in the last 3 verses,to the concept of the atma.

He advises Arjuna that nothing deserves grief,be it a body(since it has to go through various stages of birth,growth,decay,death,etc) or the atma(since it has neither birth or death and is beyond all karma).This is Krishna's jist of the Bhagavad Gita.In fact,he borrows this from the 7th chapter of the "Chaandogya Upanishad",named the "Bhuma Vidya".It is a dialogue between the guru,sage Sanatkumara and the shishya , Narada.Narada,being a very accomplished musician,having the ablility to travel anywhere as required(trilokha sanchaari),and having very special powers(siddhi),comes to Sanatkumara and says that in spite of having everything that a human being would crave for,he still experiences sorrow.He also accepts that only atma jnana can rid a person of all sorrows,and requests Sanatkumara to teach it to him.Sanatkumara agrees and his teachings form part of the 7th chapter of the Chaandogya Upanishad.
Krishna emphasizes that this knowledge alone is the solution to all sorrow.He also says that this knowledge is so profound that it is not very easy to understand.Hence,this knowledge is one of "wonder".

Why is this knowledge extra-ordinary?Three main reasons would be-

1.It talks about "Myself"."I" think "I" know myself intimately & better that anybody else.Vedanta says that I do not know myself at all.It says that I am the atma and am immortal.This I cannot comprehend now.It also says that if I am ready to undergo a very thorough educational programme,it shall convince me of the same.Hence vedanta becomes extra-ordinary,since no material object can teach me this.

2.I think that I am located at a place & am a localized individual.Vedanta challenges this and says that I,being the atma,am all pervading,and not limited by time or space.

3.I feel that the universe is very vast and that I am but an insignificant and small speck in one corner of it.Vedanta says that I am not inside the universe.On the contrary,the entire cosmos,including the space-time continuum reside in me.Sankaracharya says in the Atmabodha that "I" am an ocean in which galaxies are small bubbles appearing briefly from time to time.

Krishna says that Vedanta is too big a concept to be able to understand in a day or two,but will take years,requiring passing through the stages of Sravanam(consistent & systematic study of vedantic teaching for a length of time under the guidance of a competent acharya),Mananam(reflect in my own inetllect until I am convinced that this is not a matter of belief but that of understanding and conviction) and Nidhidyasanam(practice these to change my orientation from 'I am in this world' to 'The world is in me').This is worth the effort,because it will bring a revolutionary paradigm shift in the attitude towards oneself,towards the world and towards God.This is the only solution to all sorrow in life.

Thus Krishna says that,jnanis(enlightened people),very few are scarce in this world appreciate this knowledge.This is a wonder.
They enjoy this knowledge,and would like to spread it.Not many people are forthcoming to learn this as they are materialistically busy.When such people do come forward,the jnanis become acharyas and pass on their knowledge.This is also a wonder.

When these knowledge seekers encounter it,it is so overwhelming that they are unable to accept it right away.This is also a wonder.

The greatest wonder of all is that,even after studying these for years together,some students are still unable to understand this.