Scientism is the purported belief that science provides the only source of knowledge and understanding of the world, and the only source of value and meaning in life. Almost no-one holds this belief, and the vast majority of scientists would agree that in addition to science, there is ethics and morality, logic and mathematics, literature, and art in general. However, the development of science has progressively eroded the historical and cosmological claims which Christianity, Judaism and Islam were dependent upon, and when the combination of scientific belief and humanist morality further threatens religious dogma, the modern theist will often attempt to mis-represent scientific-humanist beliefs as 'scientistic'.
When, in 1616, the heliocentric Copernican worldview was declared to be heretical by the Catholic church, and when, in 1633, Galileo was convicted of heresy for advocating such a worldview, the Catholic church would, no doubt, have referred to the Copernican worldview as 'scientistic', if such a term had been available to them then. Heliocentrism contradicted scripture in numerous places, and such an attempt to remove man from his central position in the universe was clearly part of an attempt to extend science beyond its legitimate scope.
When a modern theist argues that science is not the only source of knowledge, the real intention is not to acknowledge the existence of ethics, philosophy and art, but to open the door to divine revelation, holy scripture, and other so-called 'spiritualities', as alternative sources of knowledge. In particular, the theist takes what is belief without evidence or reason, refers to it as 'faith', and suggests that faith offers an alternative source of knowledge, almost like a type of additional, parallel information channel to that provided by empirical, rational science. In reality, this is nothing more than an attempt to re-brand blind, ignorant belief and wishful thinking.
Another, closely allied, strategy employed by the modern theist is to argue that belief in science is itself a faith. Such claims depend upon an attempt to re-define the meaning of the term 'faith' so that a belief in anything becomes a faith. Correctly defined, as noted above, faith is belief without evidence or reason. In contrast, both scientific beliefs, and belief in the scientific method itself, are beliefs supported by reason and evidence. There is copious evidence for the success of the scientific method, and an understanding of Bayesian probability provides an understanding of why the scientific technique is so successful. Science, then, is clearly no faith.
The modern theist, wary of the success of science and humanism in Western society, is careful not to openly advocate their theism, but to construct first the paper-tiger of scientism, as an imaginary enemy, from which religion is then to provide sanctuary and salvation. On the contrary, a worldview which includes a moral system based upon rationality rather than religious decree, a scientific understanding of the physical world based upon reason and evidence, and a fully-rounded population, appreciative of the arts, philosophy and literature as well as science and technology, is the means by which the human race will be capable of progressing.