Many people subscribe to different beliefs than their faith. Many people do not understand what the doctrine of their faith even is, to be frank. About half of all Pentecostals, for example, do not subscribe to the biblical literalism that their church official subscribes to. Surveys show that even about 10% of Catholics subscribe to biblical literalism (see level of support for evolution), even if their faith officially does not. In fact, many Catholics are surprised to find out that the curia and Pope etc have issued repeated statements supporting evolution to various degrees for about 100 years. If a person wants to defend biblical literalism, depending on exactly how one interprets "literalism", it can result in an attack on evolution, which is sometimes perceived as a challenge. This is coupled with some of the apparent incredible statements of evolution; for example that we all share a lot of features with the genetic code of other living things, and that our ancesters were likely to be microbes in a primordial soup. I do not think this has to be too complicated, and does not even have to mention what denomination the person is from to any great extent, since this is more of an individual thing. It just has to be pointed out that the reason that people are attacking evolution is that they perceive it as being in conflict with a literal interpretation of the bible, and that would undermine all kinds of other beliefs they hold.