sincere vs earnest

sincere is not a noun while earnest is a noun.

Word	Noun	Adjective	Verb	Adverb
sincere	No	Yes	No	No
earnest	Yes	Yes	No	No

As adjectives, sincere and earnest are synonyms defined as:

sincere and earnest: characterized by a firm and humorless belief in the validity of your opinions

Other synonyms of sincere include solemn.

sincere (noun)	earnest (noun)
	something of value given by one person to another to bind a contract

sincere (adjective)	earnest (adjective)
open and genuine; not deceitful	not distracted by anything unrelated to the goal
characterized by a firm and humorless belief in the validity of your opinions	characterized by a firm and humorless belief in the validity of your opinions
	sincerely earnest