I've found two definitions of 'scientism' by clerics in the mid-20th Century.
In my previous post on this topic, I said that some of the early, 19th-Century adopters of the term linked it to a hubristic view of modern science as holding the promise of a complete knowledge of reality.
Here's a passage that makes this hubristic sense explicit. It's from D. R. G. Owen's 1952 book, Scientism, Man, and Religion:
We may call it scientism or scientolatry. This peculiarly modern form of idolatry refuses to recognize the limitations of science and claims that its working principles can be used as universal principles, in terms of which the whole of reality can be explained and controlled. ... Scientolatry, therefore, claims that it can solve all problems 'scientifically.' (Owen, Scientism,Man, and Religion [Philadelphia: The Westminster Press, 1952], pp. 20-21)Owen was an Anglican clergyman and Provost of Trinity College in the University of Toronto. He has here built a straw-man. Who ever claimed for science a capacity to give us the ability to 'control' everything?
Here's a better definition, which avoids imputing dreams of sweeping, comprehensive knowledge (let alone power) to those who stand for scientism. It was presented by John Wellmuth (S.J.) in his 1944 Aquinas Lecture at Marquette University:
The word 'scientism', as used in this lecture, is to be understood as meaning the belief that science, in the modern sense of that term, and the scientific method as described by modern scientists, afford the only reliable natural means of acquiring such knowledge as may be available about whatever is real. (Wellmuth, S. J., The Nature and Origins of Scientism [Milwaukee, WI: Marquette University Press, 1944], p. 5)This is not the hubristic version of scientism, for supporters of scientism are being taken to be confident about science only as the royal road to 'such knowledge as may be available' and not to an exhaustive knowledge of all reality.
The appearance in Wellmuth's definition of 'natural' is intriguing. Wouldn't the definition be better without it? Perhaps Wellmuth wanted a conception of scientism that could be applied to those religious people who allowed for divine revelation but who relied solely upon modern scientific methods for all other knowledge. If so, Wellmuth's conception of scientism positions it as a foe not so much of religion but, rather, of metaphysics.
Added (Jan. 16): I should add that my speculation re. Wellmuth's intended target was partly suggested by the focus of his Aquinas Lecture. He concentrates on medieval philosophers, esp. late medieval ones (inc. Scotus) and argues that the trend towards scientism was already under way before Descartes. It was initiated by medieval, Christian thinkers who accepted divine revelation but curtailed the prospects for metaphysical knowledge.