Unbelief, n.
Etymology: [Pref. un- not + belief: cf. AS. ungeleáfa.]
1. The withholding of belief; doubt; incredulity; skepticism.
2. Disbelief; especially, disbelief of divine revelation, or in a divine providence or scheme of redemption. Blind unbelief is sure to err, And scan his work in vain. Cowper.
Syn. -- See Disbelief.
Shades of meaning
Fernald has no entry of its own for unbelief; he lists it under doubt.
Where he writes about it:
As regards belief, while doubt is lack of conviction, disbelief is conviction, to the contrary; unbelief refers to a settled state of mind, generally accompanied with opposition of heart. — from doubt
In theological usage unbelief and skepticism have a condemnatory force, as implying wilful rejection of manifest truth. — from doubt