What is evolution? Though I am a Christian, believer in God, and staunch proponent of creationism, I believe that evolution exists. That's right, I said it... evolution is a fact of science. A better definition might be micro-evolution, or that form of evolution that takes place within a particular species. This is also known as speciation. Conversely, macro-evolution, or the change of one species to another, is bad science.
The theory that mutations of a species cause the gradual change in genetic make-up, converting one species to another, is an arduous and failing argument. Mutations are destructive. Science has never observed otherwise. All known mutations are harmful, or at best neutral. Natural selection exists, but only within the limitations of adaptability of a particular species.
This is just a single example of why it is important to define terms. Be sure you know what specific words mean, and don't forget they might mean something different to the other person.
One of the most important terms to remember is theory. Darwinism is a theory, not a fact. A theory can be defined as: a specific statement of concept, which is supported by scientific observation. Sometimes the data being observed is direct and replicable, like gravity. I can test it by dropping a ball and seeing it fall to the floor. Other times the data is circumstantial and indirect like evolution. This is the case with the origin of life. I can't watch man evolve, I can only look at circumstantial evidence and see if the observations can be fit together to support my theory.
The same is true of creationism. I didn't see God speak the universe into existence. Rather, I must formulate a theory about that and see if the observable circumstantial evidence fits. Interestingly, the theory (if we might call it that) of creationism is far more compelling than the theory of evolution. The facts of design, the fossil record, moral values, and the rest of what we have seen firmly support this position.