unbelief vs atheism

unbelief and atheism both are nouns.

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

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

Other hyponyms of unbelief include agnosticism, scepticism, skepticism.

unbelief (noun)	atheism (noun)
a rejection of belief	a lack of belief in the existence of God or gods