I tried to post this reply 3 times now...and each time something has failed and I couldn't finish the post. So, I'll try again.
What is there that you were born with, that you will never lose, and that can't be taken away from you?
It is what you were born with as a "sentinent being".
It can be called your "true nature" or "Buddha nature"....but these are only names for a thing that can't be really expressed in mere words.
As a "sentient being" you were born with this "Buddha nature" but soon your "Ego mind" began to feed you illusions and delusions to cover up your inherent "Buddha nature".
This is because your "Ego mind" wants you to believe its version of reality and feeds you these illusions and delusions to get you to accept its version of reality...so it can control you by controlling your perceptions.
But that "Buddha nature" is inherent in each "sentient being" and can never be destroyed...it may be covered with illusions or delusions, but it can mever be destroyed.
If you as a "sentient being" want to remove all those layers of illusion/delusion, you can do so.
The process can be long and difficult, but it can be done.
That however, is outside the scope of the answer to the original question
Anyhow that's my answer to the original question is....your "Buddha nature" is the one thing that you are born with, can not be taken from you, and can never be destroyed.
Shame on you Shakyamuni for setting the precedent of leaving home.
Did you think it was not there--
in your wife's lovely face
in your baby's laughter?
Did you think you had to go elsewhere (simply) to find it?
from - Judyth Collin
The Layman's Lament
From What Book, 1998, p. 52
Edited by Gary Gach