Bhagavad Gita Verse 2, Chapter 4

evam paramparaapraaptamimam raajarshayo viduh |
sa kaaleneha mahataa yogo nashtah parantapa || 2 ||
 
In this way, scholar-kings knew about this yoga through tradition, (but) through the long passage of time, this yoga has been lost here, O scorcher of foes.
 
evam : in this way
paramparaapraaptam : obtained through tradition
imam : this
raajarshayo : scholar-king
viduh : known
sah : that
kaalena : through passage of time
iha : here
mahataa : long
yogah : yoga
nashtah : vanished
parantapa : O scorcher of foes
 
Shri Krishna gives Arjuna the reason for resurrecting the Gita in this shloka. He begins by stating that this knowledge has come from teacher to student. If we trace it back to its roots, we will find that the cosmic intelligence is the origin of this knowledge. This illustrates a beautiful aspect of the spiritual tradition: no one person claims authorship of knowledge. Even Tulsidas asserted that he did not write the Ramayana, but that it came from Lord Shiva.
 
When any teaching passes from generation to generation, the original message gets distorted. Shri Krishna says that even though this knowledge was prevalent in ancient times, is become distorted due to the long passage of time. Sant Jnyaneshwar in his commentary states that the distortion was further compounded by people becoming more materialistic and extroverted over the passage of time.
 
One example of distortion is musical raagaas. The raagas that we hear today are but a distorted version of the original raagas. Another example is that of rain water. When rain water falls from the sky, it is extremely pure like distilled water. But as it falls to the ground, it gets contaminated with pollutants in the air, dust and so on. We have to process that water in order to make it potable.
 
Therefore, now and then someone has to come to refresh the original message of the Gita, and that is what Shri Krishna is doing now. But he also needs to communicate it to the right person so that the message is properly understood. He addresses this point in the next shloka.