Lord Krishna tells Arjuna, “This tradition of yoga has been there from time immemorial. I taught this to Vivasvan. Vivasvan taught this yoga to Manu. From Manu onwards Ikshvaku and other kings.” So this has been coming as a tradition from millenniums and in the due course sometime somewhere it got lost. It was not understood properly and this has been happening all along in the human history. See in a period of time, knowledge gets distorted then again it needs to be revived, rekindled, made to understand again. So Lord Krishna here tells Arjuna, “Look, this royal secret which the Gurus of the Kings always knew. I am going to reveal to you now.”
Then suddenly the doubt comes in Arjuna. Arjuna asks Krishna, “I can’t believe, Vivasvan happened a couple of millions of years ago and your life is now, you are born now. How do I believe that you taught this yoga to Vivasvan who was several thousand years ago?”
At that Krishna says, [Bhagavad Gita 4.5] “बहूनि मे व्यतीतानि जन्मानि तव चार्जुन (bahūni me vyatītāni janmāni tava cārjuna) तान्यहं वेद सर्वाणि न त्वं वेत्थ परन्तप (tāny ahaṁ veda sarvāṇi na tvaṁ vettha paran-tapa). Arjuna, for me and for you we have had many births. You have come here many times. I have come here too but you don’t remember. I remember them all.”
Though the self, the consciousness, it is all permeating, it is one. That Spirit, the self has no birth, so no death. He says, “Yes, of course I have no birth. I am the lord of the universe. Yet by my own Maya, by my own Veil, I get into the Prakritis, my own nature, and come back as an individual soul again and again and again.”
A verse of the fourth chapter is very, very famous. Everybody knows this. He says, यदा यदा हि धर्मस्य ग्लानिर्भवति भारत | 7(yadā yadā hi dharmasya glānir bhavati bhārata) अभ्युत्थानमधर्मस्य तदात्मानं सृजाम्यहम् ||(abhyutthānam adharmasya tadātmānaṁ sṛjāmy aham). “Whenever righteousness goes down or there is harm to the Dharma, ignorance rises and knowledge goes down. I come back again and again to uplift the Knowledge, to uplift the Dharma, righteousness.” So that one Consciousness which has been there as the guiding force, bringing up the knowledge, time and again, that is what Krishna was.
“Every age I am born to uplift the knowledge again.” So this is the whole concept of Avatar, descending of the Supreme Consciousness, descending of the higher Consciousness. That one Consciousness which was as a Guru to Vivasvan and then again as Shri Ram, as Krishna, then the same Consciousness in various places, in various forms have come to the planet Earth to uplift people. परित्राणाय साधुनां विनाशाय च दुष्कृताम् | (paritrāṇāya sādhūnāṁ vināśāya ca duṣkṛtām). “To uplift the good people and to destroy the bad ones.” धर्मसंस्थापनार्थाय संभवामि युगे युगे || (dharma-saṁsthāpanārthāya sambhavāmi yuge yuge). “In every age I come back to establish the Dharma back again, to establish the righteousness, to establish the good, the uplifting path again on the planet.”