Source : Webster's Revised Unabridged Dictionary (1913)

Doubter \Doubt"er\, n.
   One who doubts; one whose opinion is unsettled; one who
   scruples.

Source : WordNet®

doubter
     n 1: someone who habitually doubts accepted beliefs [syn: {skeptic},
           {sceptic}]
     2: a person who doubts truth of religion [syn: {agnostic}]