Secular alternative to "doubting Thomas"
That guy never believes anything without proof, he’s a real show-me guy.
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.
I am very much a “show me” guy and no amount of claims over the phone is going to convince me.
I'm a "science" kind of guy, a "show-me" guy, someone who tends to need some proof about claims I find questionable.
He was very much a "show me" guy. When he heard that magnesium would burn underwater he took a five-gallon coffee can to work and filled it with magnesium chips from the shop floor. He put the can in the driveway at home and filled it with water,
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.
As an alternative view to your statement I'd say that the expression doubting Thomas has been secularised in the English language from the start since there is no mentioning, as in other languages, of the "Saint" Thomas.
In French: Saint Thomas : "Je suis comme Saint Thomas, je ne crois que ce que je vois"
In Italian: San Tommaso. "Fare come San Tommaso"
In Spanish: Santo Tomás: "Ver para creer, como Santo Tomás."
What about
"That guy never believes anything without proof; he's from Missouri/a Missourian".
Someone "from Missouri" is someone who always needs proof, who always doubts. I think this option has the exact meaning you desire, and it also seems to satisfy the criterion of being "colourful".
(McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.)
- And it's well established and recognized, at least in the United States, where Missouri is nicknamed "The Show Me State". It's definitely secular, and you can find (one version of) it's origin/popularization here.
If someone doubts something, they are sceptical and therefore a sceptic
(Skeptical/skeptic in US English)