He compares our spiritual identity to salt because just as the earth has the core composition of salt, we have a core composition of spirit. Just as pure salt is salty, each of us is spiritual in essence.
We are not these physical bodies. These physical bodies are temporary and they are changing. They are made of atoms and molecules that are constantly undergoing change. Within five years we will recycle every atom and molecule. We will in effect, change bodies every five years.
Just take a look at one of your baby photos. Who is that in the photo? Is that you? It is a picture of a body, but that body is gone now. All the atoms and molecules are different from the body you wear today.
Who are we then? Who has survived through the changes in the body? It is the spiritual person within. We are each spiritual beings who are temporarily occupying a physical body.
This means our core nature is also spiritual not physical. This is why people are not satisfied by material things of this world. Because material things are not of the same essence we are.
If we act outside of our spiritual nature, we will not be acting within the context of our core character.
The spiritual character of each of us is to love, serve and please the Supreme Person, God.
If we do not live our lives true to our character—if this character is lost somehow — then we suffer the consequences of constant loneliness, emptiness and darkness.
If we do not live our lives in a spiritually-fulfilling manner - instead focusing our energies upon temporary material aspirations - then we lose this opportunity to act within our natural identity.
We in effect, lose our natural position - our "saltiness" - because we are not exchanging love with our Best Friend and Soul Mate, the Supreme Being.
(For a translation of Jesus' statements from the Gospel of Matthew without sectarian influence, see the Gospels of Jesus - translated from the original Greek texts.)