The difference between “A” and “The” is nothing to do with importance.
“The” is used when the thing being referenced is already known or is being defined (as here with “the thing”).
“A” (or “an”) is used when the thing being referenced has not been identified before.
“A dog and a cat were running away from me. The dog was black and the cat was white.”
To start with, we could be talking about any dog or cat, so it’s “a”. Once we have stated that, we know which specific animals we are talking about, so we use “the” to refer back to the dog and cat that were running.
“The elephant” – we’ve already established or are establishing which elephant.
“An elephant” – it could be any elephant; we don’t know yet.
“The elephant on the top of the hill was magnificent.”
We’re establishing that we are talking about the one on the hill (a previously-defined hill); there aren’t any other elephants.
“An elephant on the top of the hill was magnificent.”
There could be many elephants there, and we don’t know which one we are talking about.