In communicating the gospel, we must always be aware of the assumptions / presuppositions of the ones to whom we are communicating. Here's Greg Koukl on scientism...
The term “scientism” describes the view that science is the only reliable method of knowing truth about the world. Accordingly, “Everything outside of science is a matter of mere belief and subjective opinion,” says J. P. Moreland, “of which rational assessment is impossible.”
Here is how scientism self-destructs. Imagine you wanted to collect all knowledge in a box. Let’s call it the “Truth Box.” Before any alleged truth could go into the box, it must first pass the scientific truth test (the claim of scientism). The problem is that your knowledge project could never get started because some truths need to be in the Truth Box first before science itself could begin its analysis. The truths of logic and mathematics must be in the box, for example, along with the truth of the basic reliability of our senses. Certain moral truths — like “Report all data honestly” — must be in the box. In fact, the entire scientific method must be in the box before the method itself can be used to test the truthfulness of anything else.
None of these truths can be established by the methods of science, because science cannot operate in a knowledge vacuum. Certain truths — known through means other than science — must be in place before science can begin testing for other truths.
--Gregory Koukl, Tactics: A Game Plan for Discussing Your Christian Convictions