Mandukya Upanishad
With an Advaita Commentary from our Understanding
by Kenneth Jaques | 31,733 words
The Mandukya Upanishad is a short, just twelve verses, description of the material manifestation and the eventual return to unmanifest form of the Universe....
Verse 21
21. The immortal does not become mortal, nor likewise the mortal immortal. There would not be under any circumstance a change otherwise of (one's) nature.
The statement in this verse is just the logical assertion that immortal does not become mortal because that would mean it was not immortal in the first place.
Likewise that that is mortal does not have the qualities of the immortal because the immortal does not change so one cannot change to unchanging.