noun

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

he had proved all his doubters wrong

sceptic,doubting Thomas,non-believer,unbeliever,disbeliever,cynic,scoffer,questioner,challenger,nihilist,dissenter,irreligionist,atheist,non-theist,agnostic,freethinker,infidel,pagan,heathen,paynim,Pyrrhonist,nullifidian