B.G 2.12

न त्वेवाहं जातु नासं न त्वं नेमे जनाधिपाः। न चैव न भविष्यामः सर्वे वयमतः परम् ॥ २-१२॥

na tvēvāhaṁ jātu nāsaṁ na tvaṁ nēmē janādhipāḥ। na caiva na bhaviṣyāmaḥ sarvē vayamataḥ param ॥ 2-12॥

Never was there a time when I did not exist, nor you, nor these princes. Surely not, shall we all ever cease to exist hereafter.

Gīta Bhāshya 2.12

The individual being is eternal, similar to God.

How 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 'na-tve' verse explains: "The way I am famous in all the Vedas as eternal, similarly you, all the kings, are also eternal".

Gīta Tātparya 2.12

God has no destruction of the body or the self. The individual self is indestructible, but its body goes through destruction.

Because of friendship of relatives, you are forsaking your own duty (sva-dharma). Is this because of the fear of destruction of their body, or their self? There is no use fearing for the body, as it is definitely going to get destroyed. No need to fear for their being, as it is indestructible. As I am supreme being, I have no destruction. It is the same for you and everyone else as well:

"He is eternal among the everlasting entities. He is the sentient among the sentients. He is one among many. He grants everyone’s desires"–So states Vedas

"The being obtains a causal body and experiences birth and death in the form of union and disunion with its body till its liberation. But never for Hari (God)."–states Skanda Purana

Even though Krishna is Lord, because of delusion, if someone doubts eternality in Him of either body or self, it is clarified by stating 'natvevaham' i.e., "never was there a time when I did not exist".

These doubts are not with Arjuna. Even then the lord clarifies for the sake of benefiting the world. Even though this episode happened only between Arjuna and Krishna, Vyasa published it in the world.