mortal sin vs acedia

mortal sin and acedia both are nouns.

Word	Noun	Adjective	Verb	Adverb
mortal sin	Yes	No	No	No
acedia	Yes	No	No	No

As nouns, acedia is a hyponym of mortal sin; that is, acedia is a word with a more specific, narrower meaning than mortal sin:

mortal sin: an unpardonable sin entailing a total loss of grace
acedia: apathy and inactivity in the practice of virtue (personified as one of the deadly sins)

mortal sin (noun)	acedia (noun)
an unpardonable sin entailing a total loss of grace	apathy and inactivity in the practice of virtue (personified as one of the deadly sins)