Anyone who attempts to use the "Isn't evolution only a theory" argument only shows their own ignorance towards science, I'll explain why.
When talking about evolution, there are at least 3 main things you could be referring to:
1) Change in genetic characteristics in a population over time. This is a fact, it does happen, and it can be observed.
2) Historical evolution, also known as common descent, that all living things share common ancestors. This is also a fact, the evidence for it is overwhelming, through genetic, fossil and anatomical similarities.
3) The Theory of Evolution by Natural Selection.
The theory refers to the mechanism by which evolution occurs, and is distinguishable from the fact of evolution. Theories do not progress and become facts, ever. Facts do not change, theories change when more facts become available.
Here is a link to an article that explains this concept more thoroughly if anyone is interested.