B.G 2.12

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

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.

किमिति? न त्वेवाहम्। ईश्वरनित्यत्वस्याप्रस्तुतत्वाद्दृष्टान्तत्वेनाह– नत्वेति। यथा अहं नित्यः सर्ववेदान्तेषु प्रसिद्धः एवं त्वमेते जनाधिपाश्च नित्याः ॥ १२ ॥

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.

बन्धुस्नेहाद्धि त्वया स्वधर्मनिवृत्तिः क्रियते। तत्र देहनाशभयात् किं वा चेतननाशभयात्? देहस्य सर्वथा विनाशित्वान्न तत्र भये प्रयोजनम्। न च चेतननाशभयात्। तस्याविनाशित्वादेव। न तावत्परमचेतनस्य मम नाशोस्ति। एवमेव तवान्येषां च।

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:

"नित्यो नित्यानां चेतनश्चेतनानाम् एको बहूनां यो विदधाति कामान्" ॥ इत्यादिश्रुतेः।

"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

"स्वदेहयोगविगमौ नाम जन्ममृती पुरा । इष्येते ह्येव जीवस्य मुक्तेर्न तु हरेः क्वचित्" ॥ इति स्कान्दे।

"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

ईश्वरस्यापि युद्धगतत्वान्मोहात् तस्याप्युभयविधानित्यत्वशङ्काप्राप्तौ तदपि निवार्यते न त्वेवाहमिति।

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".

यद्यप्येषा शङ्कार्जुनस्य नास्ति। तथापि प्राप्तलोकोपकारार्थं भगवता निवार्यते। एकान्ते कथयन्नपि व्यासरूपेण तदेव लोके प्रकाशयिष्यति हि ॥१२ ॥

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.