Lord Kṛṣṇa says:
I taught this imperishable yoga to Vivasvān; Vivasvān taught it to Manu; Manu declared it to Ikṣvāku.

Handed down through disciplic succession, the royal sages received this knowledge. But due to the passage of time, Arjuna, this yoga was lost to the world.

It is this same ancient yoga which I now teach to you, as you are My devotee and My friend; it is indeed the highest mystery.

Arjuna says:
Your birth was recent, and the birth of Vivasvān was long ago. How is it possible that You taught this in the beginning?

Lord Kṛṣṇa says:
Many births of Mine have passed, O Arjuna, and so it is with you also. I know them all, but you do not know them.

Though I am unborn and unchanging, and although I am the Lord of all beings, by using My own nature (prakṛti), I enter into this world by My own free will.

Arjuna, whenever there is a decline in dharma (righteousness), and an increase of adharma (unrighteousness), it is then that I manifest Myself.

For the protection of the good, for the destruction of the wicked, and to establish dharma, I incarnate Myself from age to age.

Whosoever knows the truth of My divine birth and deeds, O Arjuna, is not reborn again. After leaving the body they come to Me.

Freed from desire, fear and anger, absorbed in Me, taking refuge in Me, and having been purified by the austerity of knowledge, many have attained their eternal relationship with Me.

In whatever way one surrenders to Me, in the same way I approach them. Everybody follows My path in all respects, O Arjuna.

Those who desire the fruits of their action perform sacrifices to the gods; for in this human world, results are quickly attained by such religious rituals.

I created the four varṇas based on their work and guṇas. Despite creating this system, I am still unchanging and free from action.

Actions do not taint Me, nor do I have a desire for the fruits of actions. One who understands this about Me, is not bound by karma.

Having known this, the ancient seekers of liberation also performed action, similarly you should also engage in your duty just as people in the past have done.

What is action? What is inaction? Even the wise are confused about this. I shall now explain to you what action is. Knowing this, you will be freed from suffering.

For verily one must understand what is action (karma), what is forbidden action (vikarma), as well as what is inaction (akarma). Action is indeed hard to understand.

He who sees inaction in action and also action in inaction is wise among people. He is fit for liberation and performs all his designated duties.

He whose every action is free from personal gain has his sins burnt up by the fire of knowledge. He is described by the wise as a sage.

A person who has renounced attachment to the fruits of action is dependent on no one and is ever-content in the Self. He does not act despite being engaged in activity.

Free from expectation, with a controlled mind, having no sense of ownership and acting only to maintain the body, one does not incur sin.

When one is content with what comes without effort, one has transcended the pairs of opposites, is free from envy, and is balanced in success and failure; despite acting, one is not bound.

For the liberated person, who is free from attachments, whose mind is established in wisdom, and acts only for sacrifice, karma is entirely dissolved.

For those absorbed in God, Brahman is the instrument, Brahman is the oblation; Brahman is the one who offers into the fire which is also Brahman. Brahman alone is to be reached by one who meditates upon Him in one’s actions.

Some yogīs resort only to sacrificing to the demigods, but others sacrifice into the fire of Brahman and perform sacrifice for its own sake.

Some offer hearing and the other senses into the fires of restraint. Others offer sound and the other objects of the senses into the fires of the senses themselves.

Others offer the actions of the senses and life breath (prāṇa) into the fire of yoga which is based on self-control and illumined by true knowledge.

Observing strict vows, some sages offer material objects as sacrifice, some make offerings through austerity and yoga, while others offer their scriptural study and knowledge as sacrifice.

Still there are others who offer their exhalation into the inhalation, and the inhalation into the exhalation. In this way they achieve the state where all breathing stops.

Others who restrict their eating offer the practice of breath-control (prāṇāyāma). All these yogīs know the meaning of sacrifice and through their sacrifices become free of sinful reactions.

The remnants of sacrifice is the nectar of immortality. By taking it, one attains that eternal abode of the Lord. Understand, Arjuna, that one cannot live happily in this world without sacrifice, and what can be said of the next?

In this way, many forms of sacrifice have been laid down in the Vedas. All of them are based on action; once you understand this, you shall be liberated.

The sacrifice of wisdom is superior to the ritual material sacrifice. O Arjuna, all action has its end in wisdom.

One should approach a spiritual Master with submission, extensive enquiry and service. Such realised souls can instruct you, for they have seen the Truth.

Once you receive this knowledge, Arjuna, you will never again be deluded. Through that knowledge you will see all beings, without exception, as the Self and then as part of Me.

Even if you are the most sinful of all sinners, you can cross over this miserable condition by the boat of knowledge.

Arjuna, just as a blazing fire turns wood to ashes, so does the fire of knowledge turn all karma to ashes.

In this world nothing purifies one more than knowledge. Those who have perfected the practice of karma-yoga will have this knowledge revealed to them.

One who has faith will gain this knowledge, if they are dedicated and have controlled the senses. Having attained it, one quickly realises supreme peace.

But for the ignorant, the faithless and the sceptical there is only destruction. The doubting person does not belong to this world or the next, and will never find happiness.

O Arjuna, when one is established in karma-yoga, with doubts dispelled by knowledge, having a steady mind, such a person is not bound by action.

Therefore, with the sword of knowledge, cut this doubt present in your heart, which has arisen from ignorance. Protected by this yoga, arise and fight, O Arjuna.