Part of speech: Noun
someone who claims to speak the truth
Example: a bold asserter; a declarer of his intentions; affirmers of traditional doctrine; an asseverator of strong convictions; an avower of his own great intelligence
The word "affirmer" comes from Middle English affermour.
affirm – A related word in the same word family.
er – A related word in the same word family.
Many Middle English words had multiple spellings because standardization didn't exist yet.