A doubting Thomas is a person who refuses to believe something without tangible proof, a confirmed skeptic. The term doubting Thomas comes from the New Testament story concerning the apostle Thomas. When the risen Jesus appeared to his disciples Thomas was not present, he refused to believe their story of seeing the risen Jesus. Thomas said, “Unless I see the nail marks in his hands and put my finger where the nails were, and put my hand into his side, I will not believe.” Later, when the risen Jesus appeared again, Thomas was present and recognized Jesus. The term doubting Thomas has come to mean anyone who refuses to believe something without tangible proof, usually describing someone whose lack of belief is a result of hubris.
Examples
So it is important for trustees regularly to tap their inner doubting Thomas when working with the messianic leader — is it within our scope, is the money there, can it be delivered and will its impact be assessed? (The Financial Times)
Beachcomber went to search for the boat and took pictures for the doubting Thomas at the office. (The Fiji Times)
Thomas was the first “doubting Thomas,” because he refused to believe the reports of sightings of a resurrected Jesus until, according to John 20:25, “I see in his hands the print of the nails, and place my finger in the print of the nails, and place my hand in his side…” (Haaretz)
Regarding the terrorist attack that killed nearly 3,000 and destroyed the World Trade Center, Ray is a doubting Thomas who sees the heavy hand of the government hiding key details about the actual event and withholding possible foreknowledge days preceding the disaster. (The Kane County Chronicle)