1) Christian term 2) Skeptic 
A doubting Thomas is a skeptic who refuses to believe without direct personal experience—a reference to the Apostle Thomas, who refused to believe that the resurrected Jesus had appeared to the ten other apostles, until he could see and feel the wounds received by Jesus on the cross. In art, the episode (formally called the Incredulity of Thomas...
[n] - someone who demands physical evidence in order to be convinced (especially when this demand is out of place)
a person who refuses to believe without proof; skeptic. John 20:24–29.