So... if I understand this right, having mana has nothing to do with actually being a nobel or royalty, as everyone is born with mana; other than the fact that at some point in their time, those with a lot of mana decided to differentiate themselves from those with very little mana and forgot about the gods?
I'm under the current impression that anyone in this story, considered royal, nobel, or commoner, can ultimately become Zent, under the following thoughts:
1. Very early in the story we learn all humans have mana at birth.
2. That in LN 4-7 (Investigating the Bible) the bible states anyone can increase their mana by zealot prayer to the gods. In a distant 3rd,
3. That the starbind ceremony (and other prayers) provides a hint to mana increase by suggesting prayers be given to the gods in order to gain blessings.
4. That the Grushiet is actually had by the goddess Metsionora (this is not a spoiler as it has been stated through multiple light novels already).
This would mean even commoners can increase their mana capacities, and can also be a reason why some 'commoner' children are born with 'devouring' if their parents are constantly praying to a god or to the gods for their child to be born well. It could also be why even within the royal and noble castes they vary in mana amounts even within the tiered system, ie. Mednobles having high or low mana capacities.
Is this an actual thread within this story? I am starting to think that a part of the plot is how intellectual ignorance and ego has caused the downfall the country as a whole. Though I really would like this to be the case, I wonder if I'm out in left field here or not.