unbelief vs agnosticism

unbelief and agnosticism both are nouns.

Word	Noun	Adjective	Verb	Adverb
unbelief	Yes	No	No	No
agnosticism	Yes	No	No	No

As nouns, agnosticism is a hyponym of unbelief; that is, agnosticism is a word with a more specific, narrower meaning than unbelief:

Other hyponyms of unbelief include scepticism, skepticism, atheism.

unbelief (noun)	agnosticism (noun)
a rejection of belief	the disbelief in any claims of ultimate knowledge
	a religious orientation of doubt; a denial of ultimate knowledge of the existence of God