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.