doubter

noun

A person who questions or lacks faith in something; a sceptic.

SYNONYMS

agnosticatheistchallengercynicdisbelieverdissenterdoubting Thomasfreethinkerheatheninfidelirreligionistnihilistnon-believernon-theistnullifidianpaganpaynimPyrrhonistquestionerscepticscofferunbeliever