Spoiler warning: do not read if you have not finished the entire novel.
Humans are rational beings.
As proof, humans invented the discipline known as science. An entire discipline founded on the assumption that every doubt can be formulated as a question, and that every question can be solved through experimental proof. As rational beings, we are unable to accept things without an explanation. Thus we strive to look for one.
Even before science, humans believed in gods. We relied on gods, spirits, angels, devils and mythical creatures to explain what happens around us. Most of us still believe in them. As rational beings, we are unable to accept things without an explanation. Thus, we make explanations for them. We made explanations for all the inexplicable things that occur all around us.
Even when God abandons us, we pray.
Even when calamity strikes, destroying the lives of tens of thousands of people, we pray, desperate to grab on to a final hope – some sort of explanation for the absurd loss of life, some sort of salvation from despair, anything. Aiyoku no Eustia is a story of rational beings, desperate for an explanation for their existence.
In other words, Aiyoku no Eustia is a story of faith.