earnest vs sincere

earnest is a noun but sincere is not a noun.

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

As adjectives, earnest and sincere are synonyms defined as:

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

Other synonyms of earnest include solemn.

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

earnest (adjective)	sincere (adjective)
not distracted by anything unrelated to the goal	open and genuine; not deceitful
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