In reading more about what occurred in Norway last week, it becomes clear that, whatever else was going on in Anders Behring Breivik's mind, he seemed focused on a notion of cultural/ethnic purity, obsessed with being a cultural Christian (meaning that he didn't necessarily believe in the supernatural claims of the religion, but did feel a connection to the church and the ethical code claimed by the church) and was obsessed with Europe expelling the Muslims - referring in his writings to the previous expulsions in the 11th and 15th centuries*. It would probably be best to describe his views as politically right-wing with religious overtones, and not "religious fundamentalist" as is often described.
Common in many right-wing political ideologies - especially in Europe but present in a slightly muted form in the U.S. - is an idea of an allegedly threatened ethnic white/European/Christian (which of the three is used depends on the individual) identity. The reality is, of course, that ethnicity and religion are fluid and are changing. Rather than give support to the belief that this means that they are somehow "under attack" as so many of these people claim, it is simply a statement of the realities of human history.
In the late 19th and early 20th century, before we heard of a Muslim threat to some sort of pan-European identity, we heard about German threats to French identity, Russian threats to German identity, etc. etc. Ethnicity was conceived of more narrowly, and an Italian was generally believed to be fundamentally different than a Briton. Over time, and especially after WWII (when this nationality=ethnicity view was most strongly espoused by the Nazi regime), this mellowed so that there is no a recognition of nationality and cultural differences between, say, Italy and Sweden, but it is not necessarily viewed as an ethnic difference. So, in the sense that all are considered to be of a "white European" ethnicity, or even more narrowly as regional ethnicities, the definition of European ethnicity has still changed dramatically from what it was as late as the 1940s.
Even if you are going to go the route of trying to define ethnicities as the regionally-centered complex of genetic frequencies (that is a group of people from a given area who have a higher statistical likelihood of having particular genetic traits - such as skin tone, hair color, etc.), you are still stuck trying to attach a concrete label to something that is fluid and constantly changing. If one reads the Greek or Roman accounts of western Asia, you see descriptions of people who sound as if they would be more at home in other parts of the world - tribes of red-headed middle-Easterners, blond Asian Tribesmen, etc. The reality is that many of these people belonged to groups that migrated into Europe or North Africa, or (less often) were so thoroughly crushed by the Greeks or Romans or Persians that there were few left to carry on their lineage. We are left with this notion that the genetic groupings we now see in Europe, Asia, and the Middle East or not the same as the ones that were present 1,000 or 2,000 or 3,000 years ago. They have shifted, changed, re-arranged.
Basically, no matter how you try to define ethnicity, an honest person is forced to admit that it is fluid.
What, then, of cultural identity? Many of the right-wing folks, especially here in the United States, don't talk in terms of ethnicity, but rather of cultural identity - it's okay for people to come from the outside so long as they adopt our ways, language, mode of dress, etc. etc. Isn't cultural identity under attack by allowing people to become part of our society without becoming acculturated**? There is alot to be said for the necessity of integration - where a person new to a society learns how to negotiate and contribute to that society (this requires that the receiving society allow them to integrate and that the emigrant be willing to integrate, so it's a two-way street) - but acculturation is the abandonment of one's cultural heritage in order to adopt the cultural identity of the new place wholesale...which even people who are actively trying to do it never actually seem to manage. The reality is that people move - they migrate between nations and regions within nations - and they bring their own cultural traits and ideas with them when they do. This does change the cultural identity of a region, make no doubt. But, contrary to what many people claim, this is not an assault on the existing cultural identity, which itself is nothing other than an accumulation of other such changes and additions over time. This is one of the major processes by which cultures work and change, and it has been since our early ancestors began to develop what we would understand as culture.
I don't want to get all kum-ba-yah on you. This process is not always easy, and historically has often been violent, though it is also often peaceful. But the notion that we will preserve some ethnic or cultural identity in perpetuity is absurd in the extreme. Humans never have, and even the attempt to do so itself causes changes in ethnic and cultural identity. We will change, no matter how we try to fight it. The question is, can we decide to do so in ways that benefit us?
*It should be noted that while these are often referred to as the Muslim expulsions, other groups, including the Jews, were also pushed out of various European countries during these episodes.
**Many people will use the term "integration" when they should be saying acculturation. Integration implies that the individual becomes a part of the broader society without fully shedding their previous identity - so, they learn the local language, customs, laws, etc., but they don't need to dress differently or stop speaking their native language altogether. Much of what you hear from people such as Rush Limbaugh, Sean Hannity, etc. is that they want people to "integrate" by completely abandoning their previous cultural identity rather than modifying for their present circumstances, which isn't actually integration.