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Phrase(s): doubting Thomas
someone who will not easily believe something without strong proof or evidence. (Can be said of a man or a woman. From the biblical account of the apostle Thomas, who would not believe that Jesus had risen from the dead until he actually touched the risen Christ.) • Mary won’t believe that I have a dog until she sees it. She’s such a doubting Thomas.
McGrawhill's American Idioms And Phrasal Verbs
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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).
American Heritage Idioms