Skeptic

From Conservapedia

A skeptic is one who questions the validity of a position until shown to their satisfaction that the position is warranted. The nature of what can convince a skeptic will vary from person to person. One whose philosophy on life leads them to be a skeptic by lifestyle is said to practice skepticism.

"Doubting Thomas" in the Bible is an early example of a skeptic, as he did not believe that Jesus had risen until he saw Him with his own eyes and put his fingers in the wounds on Jesus' body.