skeptic

noun

1. a person who questions the validity or authenticity of something purporting to be factual.
2. a person who maintains a doubting attitude, as toward values, plans, statements, or the character of others.
3. a person who doubts the truth of a religion, especially Christianity, or of important elements of it.
4. (initial capital letter) Philosophy.
   1. a member of a philosophical school of ancient Greece, the earliest group of which consisted of Pyrrho and his followers, who maintained that real knowledge of things is impossible.
   2. any later thinker who doubts or questions the possibility of real knowledge of any kind.