deadly sin vs mortal sin

deadly sin and mortal sin both are nouns.

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

As nouns, deadly sin and mortal sin are synonyms defined as:

deadly sin and mortal sin: an unpardonable sin entailing a total loss of grace

deadly sin (noun)	mortal sin (noun)
an unpardonable sin entailing a total loss of grace	an unpardonable sin entailing a total loss of grace