When looking at this question, the phrase "doubting Thomas" popped into my head as a potential answer. That in turn led me to question the origin of the phrase, which I discovered comes from the Apostle Thomas being skeptical of Christ's resurrection.
After learning that the phrase "doubting Thomas" has a religious origin, I'm curious…Does a "colorful*" expression that is secular in nature exist to describe a skeptical person, and if so, what is it?
Obligatory frame sentence:
That guy never believes anything without proof; he's a(n) _____________.
*By "colorful," I mean a bit more inventive/expressive than simply "skeptic," "doubter," etc.
Edit to address smci's comment: I prefer an existing and reasonably common expression; however, I will accept an uncommon or newly created expression that would be widely understood if there seems to be a consensus that a common one does not exist.
Edit to address MετάEd's comment: To further clarify and keep the question within the appropriate scope, I am not looking to compile a list of similar expressions. I'd like to identify the most common, secular expression that closely matches "doubting Thomas" in meaning and usage.
***To preserve the validity of answers that suggested coining new expressions, note that several users have asserted the lack of an expression that meets the criteria I've laid out.
Best Answer
Google finds a lot of examples of this being used in, what seems to me, the sense you are looking for and it is also easy to understand from context even if one hasn’t encountered it before.
My only reservation with this is that people do tend to put the phrase in inverted commas when they write it, which seems to set it apart as though they don't quite feel it is 'proper' language.