About the Echo (Spoilers)
As we know the Echo and the Blessing of Light are two different things. So my theory is, people with the Echo are those who contain one of the fragmented souls of one of the Ancients. When the Echo first wakes we are greeted with visions of a sky being torn apart by a meteor storm, a sky eerily similar to the Amaurot dungeon.
I know there's a few questions about this theory, the big one is why aren't there more people with the Echo?
As a civilization becomes more advanced their population tends to become smaller. Healthcare becomes better so families tend to be smaller. For a real world example, look at the population trends of American and European populations over the last thousand years.
On top of that the Ancients had a massive calamity with an unknown number of casualties BEFORE deciding to sacrifice 75% of the survivors to summon Zodiark. It's entirely possible the population of the Ancients was down to the thousands...maybe hundreds. Certainly more in line with what we see in game with the amount of Echo wielders.
But that's just my thoughts on the matter, please share yours. :D