I understand very few, if anyone, would consider Dostoevsky to be a theologian; however, his philosophy has a tremendous impact on existential theology.
In Fyodor Dostoevsky’s The Brothers Karamazov, a story of four brothers in Russia is a grim description of the reality of what the world would look like if God were not to exist. One brother, Ivan, an atheist, tells another brother that there are no objective truths, specifically that there are no moral absolutes. Ivan’s brother then kills his father, an act that obtains no condemnation if God does not exist.
This can be understood as ☐(~Eg ⊃ ∀ϕ~Wϕ), also known as Karamazov’s Theorem. It is necessarily true that if God does not exist then any action cannot be wrong. It may also be true if a conjunct of rightness is inserted into the theorem. This ultimately leads to moral nihilism—a nonexistence of value. Without God, everything is permitted. Nothing can be praised and nothing can be condemned. This world, as Dostoevsky understands it, is a world of nothingness.