Unbelief in the middle-to-late 19th century began to take up the profound new sense of the universe, its vastness in space and time, and in the lack of a plan.
In a philosophical debate, there is an implicit burden of proof on the party asserting a claim, since the default position is generally one of neutrality or unbelief.
In the old world, people could have a naive belief, but today belief or unbelief is reflective, and includes a knowledge that other people do or do not believe.
Unbelief, insofar as it is not a pure absence of religious questions or interests, is equally a response to the question addressed to humankind by religion.