I don't understand what's unclear with this question.
The comment says:
Wait... what is scientism exactly? It sounds like a very particular form of philosophy. Is scientism related to Christian Science or Theosophy?
But in case that was unclear, I already explained in my own question what is scientism!
Is, therefore, the correct term Scientist, or is there any more precise term?
"A ___ believes that science is the only legitimate source of knowledge."
As opposed to:
"A scientist has won the Nobel prize for discovering a vaccine against cancer."
If it is still unlcear, here is the relevant Wikipedia link: https://en.wikipedia.org/wiki/Scientism
So, in summary: Scientism is, roughly speaking, the belief that the only legitimate knowledge is scientific knowledge and all other sources of knowledge - like religion - should be rejected.
Now in my language (Polish) there are two distinct terms: "naukowiec" - which means a scientist (the person who conducts scientific research and who can win a Nobel prize for discovering a vaccine against AIDS) and "scjentysta" - which means a person whose worldview is in accordance to scientism (so "scjentysta" will reject all arguments that are not scientific in nature and will condemn religion).
But, how to say "scjentysta" in English? Since what I want is a believer of scientism, I would say "scientist" BUT - this will cause confusion since not every practicioner of science believes in scientism and not every person whose worldview is in accordance with scientism will have PhD.
So, is my only option still "scientist" or is there any better word?