Ceruleum is liquid condensed aether, that will readily turn gaseous if exposed to air - at least this is the largely accepted theory I've seen. So they already have plenty of it in supply all around them at any given time. The only problem is whether we consider the Garleans Atheist or Agnostic. Agnostic is more true towards the philosophy of not believing in a god, where-as atheism is a firm belief that there is no god. They are not allowed to believe in a god, but that doesn't necessarily mean they believe there is no such thing as a god. That component of faith is important, as it would be considered a direct faith in the power of the Emperor. The next question is, can Garleans summon a primal? They do not possess the ability to channel the aether, so can they actually transfer their faith to a focus? We learned this week that the limit on magic is actually not tied to the body. Something intrinsically built into a Garlean's soul is actually what prevents them from channeling the aether. (Elezenos may not be able to be used as an opposing case study, as the Resonant transfered with his soul, meaning it was a metaphysical graft/augmentation to his soul, not his body. Thus, magic may be attributed to the Resonant if he uses magic as an Elezen). From this same line of thought, I suppose this could be worked around by installing the Resonant in many devout servants of the Empire.

However, after King Thordan, this really needs to start bringing into question if the Warrior of Light may end up being host to a summoning. We are a veritable fount of aether at this point, and we have the absolute faith and prayers of "some half dozen nations" as Ser Aymeric put it. If I recall, Elidibus once stated that we are toeing the line into the realm of the gods already. King Thordan himself displayed a very valid reason for why we might even want to take a summoning unto ourselves. "I require aether. It does not matter the source." as King Thordan demonstrated that the very nature of a primal actually hard counters the eternal nature of the Ascians by consuming their aether. Furthermore, Warriors of Light taking this burden was already done, as spoken in the Warring Triad storyline. The Warriors of Light of the 13th Shard had taken on the powers of primals, and eventually grew mad in their thirst for power, eventually consuming their world in eternal darkness, and created the Void. Return to Ivalice, we learn that Warriors of Light of old had used Magicite to basically have "Primal in a Can", and become primals themselves for short, controlled spurts. The context and precendent for we as Warriors of Light currently exists, and I fully believe this will one day happen. It would work well in a Warrior of Darkness theme, but from what Yoshi-P has said about what we're exactly doing in Shadowbringers, it's easy to defer that we're just not going to be doing anything remotely "morally grey" still. So, I doubt it will happen in Shadowbringers.