affirmer vs postulator

affirmer and postulator both are nouns.

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

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

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

affirmer (noun)	postulator (noun)
someone who claims to speak the truth	someone who assumes or takes something for granted as the basis of an argument
	(Roman Catholic Church) someone who proposes or pleads for a candidate for beatification or canonization