being vs eternity

being and eternity both are nouns.

Word	Noun	Adjective	Verb	Adverb
being	Yes	No	No	No
eternity	Yes	No	No	No

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

being: the state or fact of existing
eternity: a state of eternal existence believed in some religions to characterize the afterlife

being (noun)	eternity (noun)
a living thing that has (or can develop) the ability to act or function independently	a state of eternal existence believed in some religions to characterize the afterlife
the state or fact of existing	time without end
	a seemingly endless time interval (waiting)