Well, the term/movement "Creationism" is relatively new, but the ideas are most definitely NOT. People have been believing that Yahweh created the world in six days since ancient Israel. Creationism predates Christianity itself. It's just that there was no need to label it as such since there was no science to oppose it. It seems to be split about half way. ~80% of Americans identify as Christian. ~40% of Americans are creationists. I think the more serious a Christian is about his or her faith, the more likely they are to be a creationist and there is a reason for this. The fall of man is foundational to Christian doctrine. If man is simply another animal that evolved from a single celled organism over billions of years, then the whole worldview painted by the Bible falls apart. Christians who accept Evolution just aren't very serious and aren't thinking it through (well the creationists aren't thinking it through either but in a different way). Lots of Christians believe in Evolution but that doesn't legitimize the position.