Quote Originally Posted by Brightamethyst View Post
Soulstones are not required to become a job (except SCH and WHM.)

Soulstones are a history of the job and the abilities learned by the previous holders, making it easier for the current holder to learn said abilities. It's possible to become a paladin/bard/dragoon/etc through time and effort and training alone. The soulstone is just a shortcut that lets you skip the years of extra training. (The exceptions being SCH where the fairy is tied to the stone, and WHM where the elementals won't accept a non-padjal without it.)
Principia also says a soul crystal is required to become a Summoner. He doesn't explain why, but he lists it as one of the criteria.

I don't even want to think about the sheer confusing mess that is the Paladin soul crystal, since apparently both the writers and the NPCs want to forget that the Heavensward "Paladin soul crystals must fight for dominance" questline even existed.