The question of the place of science in knowledge, and in society, and in life, is not a scientific question. Science confers no special authority, it confers no authority at all, for the attempt to answer a nonscientific question. It is not for science to say whether science belongs in morality and politics and art. Those are philosophical matters, and science is not philosophy, even if philosophy has since its beginnings been receptive to science. Nor does science confer any license to extend its categories and its methods beyond its own realms, whose contours are of course a matter of debate. The credibility of physicists and biologists and economists on the subject of the meaning of life—what used to be called the ultimate verities, secularly or religiously constructed—cannot be owed to their work in physics and biology and economics, however distinguished it is. The extrapolation of larger ideas about life from the procedures and the conclusions of various sciences is quite common, but it is not in itself justified; and its justification cannot be made on internally scientific grounds, at least if the intellectual situation is not to be rigged. Science does come with a worldview, but there remains the question of whether it can suffice for the entirety of a human worldview. To have a worldview, Musil once remarked, you must have a view of the world. That is, of the whole of the world. But the reach of the scientific standpoint may not be as considerable or as comprehensive as some of its defenders maintain. (Source)
Scientism isn't science. Science is important for giving us explanations about how the world works mechanically, but close to useless for explaining why or for what purpose it does so. Science answers 'how' questions, not 'why' questions, and I'd argure that science is on shaky grounds even to answer 'what' questions. Science remains agnostic to 'why' and 'what' questions, and stops being science as soon as it begins to extrapolate.
'Scientism' is that extrapolation. It takes the materialistic and mechanistic dimension of our experience, and projects it outward as if it provides an especially authoritative explanation. For some this explanation has 'authority' because it is fact based, but what passes for fact is at best a provisional interpretation based on a body of knowledge that is open to reinterpretation when new 'facts' are added. For me the advocate for scientism is like the colorblind man who lives in a grayscale world and finds reports of color nonsense.
For the life of faith is at least in part a taking seriously the reports of people who say there is a world beyond what the senses can know. (That is not all it is, but it starts that way.) And so the world divides into people who take seriously those reports and those that don't. There are many reasons, good and bad, for taking those reports seriously, but here's a simple test. What is the effect of one's hearkening to those reports? It is good if it has an expansive effect that cracks the shell that encloses us in small worlds or our own making. It is bad if it simply creates another shell that traps us in a small, safe world.
Those who embrace a religious fundamentalism and those who embrace a materialistic scientism are mirror images of one another insofar as both are trapped in mental structures constructed to keep them safe in world made up of manageable, grayscale certainties.