mortal sin vs deadly sin

mortal sin and deadly sin both are nouns.

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

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

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

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