declarer vs postulator

declarer and postulator both are nouns.

Word	Noun	Adjective	Verb	Adverb
declarer	Yes	No	No	No
postulator	Yes	No	No	No

As nouns, postulator is a hyponym of declarer; that is, postulator is a word with a more specific, narrower meaning than declarer:

declarer: someone who claims to speak the truth
postulator: someone who assumes or takes something for granted as the basis of an argument

declarer (noun)	postulator (noun)
the bridge player in contract bridge who wins the bidding and can declare which suit is to be trumps	someone who assumes or takes something for granted as the basis of an argument
someone who claims to speak the truth	(Roman Catholic Church) someone who proposes or pleads for a candidate for beatification or canonization