In the most important sense a creationist is a person who believes in creation, and that includes people who believe that Genesis is a myth and that creation involved a process called evolution and consumed billions of years.
As a theist I believe that God exists and that God creates.
Most importantly, I agree that the truth of these matters should be determined by interpretation of scientific evidence - experiments, fossil studies and the like.
In short, the proposition that God was in any way involved in our creation is effectively outlawed, and implicitly negated.
If modernist naturalism were true, there would be no objective truth outside of science. In that case right and wrong would be a matter of cultural preference, or political power, and the power already available to modernists ideologies would be overwhelming.
According to the scientific naturalist version of cosmic history, nature is a permanently closed system of material effects that can never be influenced by something from outside - like God, for example.