Belief is a form of imagination which is not based on evidence. Belief is not reasonable, not factual. Belief, by the way, includes not only religious belief, but also political beliefs, ideologies, and all kinds of concoctions and habits usually called "superstition" and "magical thinking".
Belief in all its forms is made possible because at its basis is a complex of interrelated beliefs, formed during human evolution and assimilated during childhood, which I will call here "core beliefs".
The belief in an identity separated from knowing. An "I" or "ego" which centralizes itself in an "inner world" and simultaneously projects a "world out there".
The belief in "objects" through the objectification and naming of the flow of impressions into separate, resistant things, both psychic (thoughts, feelings) and physical objects.
The belief that the imagined self is powerful, responsible and capable of manipulating the flow of experience, meaning that the self has "free will".
The belief in transient time, with a causal past and a future that can be planned.
Presumably, the fundamental beliefs are the result of evolution of the human animal and this evolution is re-experienced during the upbringing of the human infant.