disbeliever

ˌdɪsbɪˈliːvə

noun

a person who refuses to believe something or who lacks religious faith.

she intends to prove the disbelievers wrong

unbeliever,non-believer,atheist,non-theist,irreligionist,nihilist,rationalist,sceptic,doubter,agnostic,doubting Thomas,questioner,challenger,cynic,scoffer,nullifidian