What is the difference between "belief in God is just wishful thinking“ and "belief in God is..."?

Wishful thinking is a category of mental error, and does not require an article ('a') before it. We never say "a wishful thinking".

Con is quite right though occasional rare instances arise when one might use "a"

I asked of my Better Half, who is much smarter than I, and she replied that in most cases the use of "a" is wrong, with the possible exception of subject "...narrow, one particular point"

For instance John Edemeir's conclusion in the matter of the life of an electron, in spite of assertions of denial by other experts in the field, his hope to learn the effect of a change in its velocity according to its age, might be called a wishful thinking

I'd rather call it a 'wishful thought' or maybe better 'an example of wishful thinking' although I would be uneasy about this because the phrase 'wishful thinking' has a particular meaning: "interpretation of facts, actions, words, etc., as one would like them to be rather than as they really are" or "the formation of beliefs and making decisions according to what might be pleasing to imagine instead of by appealing to evidence, rationality, or reality." If the nature of the electron as asserted by Edermeir's detractors is not a proven fact, 'wishful thinking' would seem to be inappropriate, as there is not a 'reality' that he is wilfully denying.

Who is this Edemeir character? Google doesn't seem to find him.

Yea Con that thought had occurred to me also

Yea even better but I was stuck with "a wishful thinking"

It's the assertions of denial that he could be willfully denying. Maybe somebody else hereabout could dredge up a better example than mine

I'm impressed by your curiosity and flattered that you would attach probability to his assertions. It's entirely fiction

Thank you both.

Here's another "a" question:

Which is correct?
"The passage therefore preserves uniquely early and verifiable testimony. It meets every reasonable demand of historical reliability."
versus:
"The passage therefore preserves a uniquely early and verifiable testimony. It meets every reasonable demand of historical reliability."

Would seem to imply there might have been a single testimonial by an especially important character