Source: Roget’s New Millennium™ Thesaurus, First Edition (v 1.2.1)
Main Entry: doubter
Part of Speech: noun
Definition: nonbeliever
Synonyms: agnostic, cynic, disbeliever, doubting Thomas, headshaker*, questioner, skeptic, unbeliever, zetetic
Antonyms: believer, worshipper
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* = informal or slang
Space Enthusiast (formerly Cynic) 