Unbelief, disbelief, incredulity are comparable when they mean the attitude or state of mind of one who does not believe.
Unbelief stresses the lack or absence of belief especially in respect to something (as religious revelation) above and beyond one’s personal experience or capacity.
Disbelief implies a positive rejection of what is stated or asserted.
Incredulity implies indisposition to believe or, more often, a skeptical frame of mind.