Dictionary:

doubting Thomas

  (dou'tĭng)

One who is habitually doubtful.

[After Saint THOMAS, who doubted Jesus's resurrection until he had proof of it.]

noun

One who habitually or instinctively doubts or questions: doubter, nonbeliever, skeptic, unbeliever. See belief/unbelief.

One who is habitually doubtful. For example, He was a doubting Thomas about the coming merger, not believing it would ever happen. The term alludes to the disciple Thomas, who doubted Jesus's resurrection until he had first-hand evidence of it (John 20:24-29).

The noun has 2 meanings:

Meaning #1: the Apostle who would not believe the resurrection of Jesus until he saw Jesus with his own eyes
  Synonyms: Thomas, Saint Thomas, St Thomas, Thomas the doubting Apostle

Meaning #2: someone who demands physical evidence in order to be convinced (especially when this demand is out of place)