The truths held by science are not "just theories", they are "scientific theories", which is something totally different. You can take my word on this by "faith" or read a little about this. Science has not much to do with faith, but rather with facts. "Scientific theories describe the coherent framework into which observable data fit. The "theory of evolution" is the framework that best explains observed changes of species over time and best predicts the new observations that continue to be made in evolutionary biology and related sciences. The scientific definition of the word "theory" is different from the colloquial sense of the word. Colloquially, "theory" can mean a conjecture, an opinion, or a speculation that does not have to be based on facts or make testable predictions. In science, the meaning of theory is more rigorous: a theory must be based on observed facts and make testable predictions. In science, a current theory is a theory that has no equally acceptable or more acceptable alternative theory, and has survived attempts at falsification. That is, there have been no observations made which contradict it to this point and, indeed, every observation ever made either supports the current theory or at least does not falsify it by contradicting it completely. A revision of the current theory, or the generation of a new theory is necessary if new observations contradict the current theory, as the current findings are in need of a new explanation (see scientific revolution or paradigm shift). However, the falsification of a theory does not falsify the facts on which the theory is based." Source and further information: http://en.wikipedia.org/wiki/Evolution_as_theory_and_fact

people need something fictional to believe in.