Consider the following two propositions:
1) We know from the faith that the earth is the immobile center of the universe, unmoved, about which the universe rotates.
2) We know from science that God is an unnecessary entity; we can account for the formation of the universe and all physical phenomena without positing his existence.
As polarized as these two statements are, they represent the mirror image of fideism and scientism.
Scientism has been discussed quite well in a recent article titled "Scientism, Knowledge, and Truth," and there is not much for me to add. Suffice to say that scientism, which I have discussed on this blog before, is a peculiar habit by which some scientists and other personalities affirm the methodology and findings of modern natural science as a self-contained, universal philosophy. Science is seen, at least to some degree if not in its totality, as its own context and justification, bounded only by whatever moral and ethical concerns can be put forward by the secular state in which it is operating. The descriptions and explanations of physical phenomena put forward are posited as "cold, hard facts," indisputable statements which are meant to form the foundation of a constructivist attempt at formulating meaning, guiding politics, and informing individual lives. It is not necessarily that a proponent of scientism rejects anything not "scientific," but rather that science becomes the predominant, if not only, tool by which ideas and propositions can be validated.
The problems with this view are manifold. The most immediately apparent is that it deforms the methodology of science and ignores one of the most important aspects of that methodology: modern science can only speak on hypotheses regarding observable physical phenomena which are falsifiable either through experimentation or through the observation of a theorem's predictions. Science performs this task admirably, so much so that the beginnings of modern science shook the established order so thoroughly that one of its early proponents, Galileo, was placed on trial by the Inquisition to challenge his novel idea that the earth orbited the sun. But its power comes from the relatively narrow scope of its methodology. Science may be able to prove whether two objects of different mass fall at the same speed in a vacuum. It cannot tell us what Shakespeare meant to teach us - if anything - in "Measure for Measure." Though science can provide an indispensable service to various arts and other areas of human endeavor, it cannot take over their roles, because a great deal of meaningful human existence consists of propositions, stories, and narratives that are not falsifiable through experimentation.
Science requires a context of meaning, and that context will always consist to some degree of extra-scientific assumptions drawn from culture. Scientism, by rejecting this principle, not only obscures the role of science, but it also becomes a self-deceiving project which must work constantly to obscure the extra-scientific assumptions at its core.
But if scientism is the distorting of science into an all-encompassing philosophy, fideism is a mirror image error which distorts faith into something equally malformed. Fideism is the error by which the proponents of a faith (in my case, I will assume Catholicism) construe that faith (at least, faith as they understand it) is an all-encompassing and complete, incorrigible body of knowledge that supersedes all findings of reason. The temptation to fideism and the temptation towards scientism are both ultimately a desire for a complete, circumscriptive comprehension of all reality under a single unimpeachable system.
Fideism is dangerous to true faith because, as Catholics understand, our understanding of the faith is always subject to growth. While the core and fundamentals of faith have been universally present throughout the history of the Church, it is of no doubt that our understanding of those fundamentals have undergone a kind of evolution, in which principles once obscure or uncertain are gradually brought to more and more certain light. In some cases, especially in the case most relevant to this particular point, the inquisition of Galileo, the touchstone for a re-examination of a then contemporary interpretation of the faith - an interpretation that required a painfully literalist hermeneutic - was actually the findings of science. While it is clear that science and faith have separate "zones of competency," it is also the case that both sides have at times stepped beyond their own competency. Faith, the body of revealed truth given by Christ and handed down under the teaching authority of the Church, represents the knowledge of God, man, and creation necessary for salvation. It does not, then, necessary to believe that it means to teach us about the physical structure of the cosmos or the presence of some preferred frame of reference.
What is needed, then, is a complementary view of faith and science which understands their unique competencies as mutually correcting. Faith, as an integral part of the culture in which science operates, is also an indispensable part of the context for the findings of science. While it is certainly true that science can operate without theistic or other religious assumptions - and, in fact, there is a real sense in which it must operate without these assumptions in order to fulfill the requirements of its own methodology - it is also true that the meaning of these conclusions are not self-evident from the empirical method but must be interpreted with the guide of culture and extra-scientific narratives. Similarly, science's power to teach us about the physical structure and history of the cosmos offers faith a correction to overly literalist interpretations of scriptures, as well as providing insight into the human person as a body. These insights are necessarily of interest to the faith, which should always be watchful lest we condemn the next Galileo without first considering the possibility that we have misunderstood the gift which Christ has given us.