B.G 2.12

Never was there a time when I did not exist, nor you, nor these kings. Surely, nor will there be a time in the future when we will all cease to exist.

Gīta Bhāshya 2.12

The way I, i.e. Lord Krishna, am famous in all the Vedas as eternal, similarly you, all the kings, are also eternal.

Why so (why should not one worry)? 'na-tvevaham' verse gives the answer. There is no need to prove the eternality of God (Ishwara). Taking Him as an example, the 'na-tve' verse explains: "The way I, the God, am famous in all the Vedas as eternal, similarly you, all the kings, are also eternal".

Gīta Tātparya 2.12

Because of attachment to relatives, fearing their destruction, Arjuna is deviating from his 'sva-dharma', his own duty as a warrior. The fear could arise because of the destruction of the physical body or because of the destruction of conscious-being stationed within the body. Krishna clarifies that there is no need to fear the destruction of the physical body, as it is unavoidable anyway. There is no reason to fear the destruction of conscious-being within, as it is indestructible. He takes the example of himself, the supreme being, the God, and says, "Just as there is no destruction to Me, there is no destruction to you or others". The Lord is famous in the Vedas as the eternal among eternals and the conscious among the conscious-beings.

Indeed, it is due to attachment to relatives that you cease from your own duty, the sva-dharma. Is this because of fear of the physical body's destruction or the destruction of conscious-being? Since the body is inherently destructible, there is no reason to fear. Nor should there be fear of the destruction of conscious-being, as it is indestructible. There is no destruction of my supreme consciousness. Likewise, it is the same for you and others.

"He is the eternal among eternals, the conscious among the conscious-beings, he is the one who fulfils the desires of the many."

- stated thus in the Vedas.

"The cycle of birth and death, the union and separation from the body, and the liberation are referenced for ordinary beings; but it never applies to Lord Hari."

- stated thus in the Skanda Purana.

Even though Krishna is Lord, because of being involved in battle, from delusion, if someone doubts eternality in Him of both kinds, it is clarified by stating 'natvevaham', i.e. "never was there a time when I did not exist".

These doubts are not present in Arjuna. Even then, the Lord clarifies for the sake of benefiting the world. Even though this episode happened privately (only between Arjuna and Krishna), in the form of Vyasa, he published it to the world.