Christianity is a very prevalent religion in the western world, to a greater or lesser extent. In Ireland, as we are predominantly Roman Catholic, it is more of a background thing; it's there, but not really obtrusive.
Creationism is... a section, I suppose, of Christianity, born out of the need to hold the Bible to be the true and absolute Word of God. It holds that every story, every detail in the Bible is literal, including the passages dealing with the creation of the world and the origin of life on earth - hence the name. This puts it at odds with the scientific understanding, that states that the earth is old and life evolved.
Ah, the theory of evolution. I doubt any theory has attracted as much attention from an organised religion, and as much misunderstanding. At heart it is an explanation, based on evidence gathered over years of study, of how life came to be the way it is now. Scientists consider it to be a very good explanation - Darwin proposed the basis of it in 1859 in On the Origin of Species, and new genetic evidence discovered in more recent years confirmed it, when it could have easily disproved it completely.
That, of course, is the true test of a theory - whether it makes firm predictions about future evidence, and whether those predictions are shown to be right. And a theory, in science, is in fact the complete opposite of a 'theory' as used in general speech - it is something so well supported that in earlier times it would have been called a Law, like Newton's Laws of Motion.
Why would creationists deny it? Evolution directly contradicts the story of Genesis, that states that all the species on earth were created by God. By their reasoning, it cannot be true.
I find this interesting because science does not hold theories to be 'true' in the way that religions would regard their holy texts to be true. Nothing is proven beyond all doubt in science; everything is tentative, meaning that if evidence were discovered that showed that evolution could not have happened, evolution would be discarded. So it is well supported, by multiple lines of evidence, but also very fragile - if a human skeleton was found in the same rock as a dinosaur, evolution would immediately fail. The almost amusing situation here is that creationists hold that evolution is not true - but in a way, science doesn't either!
There is a level of antagonism here on the part of creationists that seems misplaced. Evolution as it stands is only an explanation, after all - it is useful, predictive, and unifying, but that's all it is. (Scientists, of course, will laugh at me for that - 'all it is', is something quite powerful, but let's leave that aside for a moment!) Creationists may state firmly that evolution cannot be true, but that ignores how useful it is as an explanation. In order to supplant it, creationists must develop a new explanation that is just as useful, predictive, and unifying - not an easy thing to do, but the one to do it will probably become famous overnight. In science, disproving a theory is very exciting.
I may write more on creationism. I find it fascinating as a concept.