मृत्युप्रोक्तान्नचिकेतोऽथ लब्ध्वा विद्यामेतां योगविधिं च कृत्स्नम् ।
ब्रह्मप्राप्तो विरजोऽभूद्विमृत्युरन्योऽप्येवं यो विदध्यात्ममेव ॥ १८ ॥

mṛtyuproktānnaciketo’tha labdhvā vidyāmetāṃ yogavidhiṃ ca kṛtsnam |
brahmaprāpto virajo’bhūdvimṛtyuranyo’pyevaṃ yo vidadhyātmameva || 18 ||

18. Nachiketas then having acquired this knowledge imparted by Death and also all the instruction about yoga, attained Brahman, having become free from taint and death; so does another also, who thus knows the nature of the atman.

Shankara’s Commentary:

This conclusion of the story intended to eulogise knowledge is now stated. Nachiketas having obtained from Death by the granting of boons, the knowledge of the Brahman imparted as above said and also all the instruction about yoga with its fruits, attained Brahman, i.e., became immortal. How? Being already freed from vice and virtue and from desire and ignorance. Not merely Nachiketas alone but even another who like Nachiketas, knows the atman and realizes its true nature unconditioned and within all, and not its form which is not within all. Who knows the nature of the atman, as thus stated, such knower also; ‘being untainted becomes deathless, by attaining Brahman’ should be supplied to complete the sentence.