Verse 2.18: The material body of the indestructible, immeasurable and eternal living entity is sure to come to an end; therefore, fight, O descendant of Bharata.
The past few and upcoming verses specifically deal with the characteristics of the soul. Here we are told that it is indestructible, immeasurable and eternal. You know, it's interesting how we as a society are so preoccupied with the external. In our day to day lives, we are bombarded by descriptions and advertisements of the material body - beautiful people with the right body, hairstyle, clothes, makeup, accessories etc., and are told to run after it. Our knowledge of how to make our physical "look good" is far advanced, but what about the soul?
Whoever asks about descriptions of the soul? Very few people. In fact, most would be hard pressed to even name a scripture or text that gives specifics of the nature, size and qualities of the soul. This is why we are so lucky to have the bhakti texts. As inquisitive beings, it is not satisfying to hear vague answers, especially about important topics.
We know so much about the body, which we have mistakenly identified with for millions of lifetimes, so isn't it just as natural that we should become equally if not better versed about who we really are? Just as each of us know our eye colour, hair colour, size of clothing and physical capabilities of the machine we are currently operating (the body), similarly we should just as easily be able to list all the qualities that define the soul which is operating that machine.
This verse is educating us on the qualities we really should associate ourselves with. The soul can never die by any means. That is the actual meaning of indestructible. It's so strong. Something to take comfort in when we are feeling weak. The soul cannot be measured. The Gita gives descriptions of how small the soul is, but it cannot be measured by any material means, hence it is immeasurable. And finally, the soul is eternal. That means we do not need to fear death because we will never die. The machine, the external coverings we are currently inhabiting may wither away and cease to function, but we, the soul, will never die.
These three characteristics can give the bhakti yogi so much hope and strength. Self-help books often say that "We should look to see the positive qualities within ourselves". Well, I doubt anyone can find more comforting and empowering qualities than these ones. So remember this, as you continue your journey as a spiritual warrior. All the strength you are searching for is truly within.