Asseverator is a noun commonly used to describe someone who passionately declares or asserts something to be true. However, there are various synonyms that can be used interchangeably, such as affirmant, asserter, proclaimer, declarer, avower, and assertor. Each of these words carries slightly different connotations but ultimately convey the same sense of claiming something to be factual or true. Other synonyms that might be a little less commonly used include avoucher, vouchsafer, or testimonialist. Regardless of which word you choose to use, they all convey the same sense of confidence and conviction in the statement being made.