How often have you heard theists say this? Personally I hear it at least 5 times a week. while walking around my college campus with atheist and evolution shirts I'm often stopped and asked about my belief. example one shirt I have says " Fine, I evolved, you didn't", so sometimes I spark quite interesting discussions on campus. Now as a scientist I am often insulted when people say "Evolution is only a theory" for 2 reasons.
1) They don't actually know what a theory is.
2) Because they don't know what a theory is, they don't recognize that theories are more important than facts.
Now the scientific definition of a theory is "the analysis of a set of facts". Meaning logical, observable and scientific procedures are used to conclude based on FACTS that a method should be called a theory. Evolution is a theory, yes, but that just means it is several facts put together in a logical format. So the next time a christian or theist says "evolution is only a theory" simply ask them what a theory is, then explain to them what it actually is.