B.G 2.12

na tvevāhaṃ jātu nāsaṃ na tvaṃ neme janādhipāḥ। na caiva na bhaviṣyāmaḥ sarve vayamataḥ param॥

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 all will cease to exist.

Gīta Bhāshya 2.12

Similar to God, the individual being is also eternal.

kimiti? na tvevāham। īśvaranityatvasyāprastutatvāddṛṣṭāntatvenāha– natveti। yathā ahaṃ nityaḥ sarvavedānteṣu prasiddhaḥ evaṃ tvamete janādhipāśca nityāḥ ॥ 12 ॥

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 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.

bandhusnehāddhi tvayā svadharmanivṛttiḥ kriyate। tatra dehanāśabhayāt kiṃ vā cetananāśabhayāt? dehasya sarvathā vināśitvānna tatra bhaye prayojanam। na ca cetananāśabhayāt। tasyāvināśitvādeva। na tāvatparamacetanasya mama nāśosti। evameva tavānyeṣāṃ ca।

Indeed, it is due to attachment to relatives that you cease from your own duty, '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, due to its indestructibility. So much, there is no destruction of my supreme consciousness. Likewise, of yours and others.

"nityo nityānāṃ cetanaścetanānām eko bahūnāṃ yo vidadhāti kāmān॥"

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

ityādiśruteḥ।

- stated thus in the Vedas.

"svadehayogavigamau nama janmamṛtī purā । iṣyete hyeva jīvasya mukterna tu hareḥ kvacit॥"

"The cycle of birth and death, the union and separation from the body, is something that is sought for the liberation of ordinary beings, but it never applies to Lord Hari."

iti skānde।

- stated thus in the Skanda Purana.

īśvarasyāpi yuddhagatattvānmohāt tasyāpyubhayavidhānityatvaśaṅkāprāptau tadapi nivāryate na tvevāhamiti।

Even though Krishna is Lord, because of 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".

yadyapyeṣā śaṅkārjunasya nāsti। tathāpi prāptalokopakārārthaṃ bhagavatā nivāryate। ekānte kathayannapi vyāsarūpeṇa tadeva loke prakāśayiṣyati hi ॥12॥

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.