sceptic

Concise Oxford English Dictionary © 2008 Oxford University Press:
sceptic /ˈskeptɪk/ (archaic & N. Amer. skeptic)

▶noun

• 1 a person inclined to question or doubt accepted opinions.

  ■ a person who doubts the truth of Christianity and other religions; an atheist.

• 2 Philosophy a philosopher who denies the possibility of knowledge, or even rational belief, in certain spheres.

– derivatives
sceptical adjective,
scepticism noun.

– origin C16: from Fr. sceptique, or via L. from Gk skeptikos, from skepsis ‘inquiry, doubt’.