departed vs at rest

departed is a noun but at rest is not a noun.

Word	Noun	Adjective	Verb	Adverb
departed	Yes	Yes	No	No
at rest	No	Yes	No	No

As adjectives, departed and at rest are synonyms defined as:

Other synonyms of departed include asleep, at peace, deceased, gone.

departed (noun)	at rest (noun)
someone who is no longer alive

departed (adjective)	at rest (adjective)
dead	in a state of repose or especially sleep
well in the past; former	dead