Every third member of the Norwegian Church is an atheist, and only 48 percent consider themselves to be Christians, according to a new poll.

In the survey made by Response Analysis for Aftenposten, 40 percent of the entire population say that they regard themselves as Christians.
Helga Haugland Byfuglien, permanent president of the Bishops’ Conference, is not surprised by the result.

– These figures line up with a number of other surveys. The proportion who are members of the Norwegian Church has decreased. The main reason for that is that the make-up of the population has changed. The number of members is fairly stable, but we recognize that we as a church are challenged by new trends, she says.

Among those over 60 years 53 percent of the population reply that they consider themselves to be Christians, while only 22 percent under age 30 say the same.

Source

Here is what I believe is the main reason why;

Citizens born in Norway to one or two Norwegian parents are automatically added to the list of Protestant Christians in Norway, and are required to "sign out" of the church. There are two categories kept in the church's books, "medlemmer" (members) and "tilhørige" (belonging to [the State church]). Members technically have to be baptised, whereas "tilhørige" are to be taken out of the books if not baptised by the age of 18. Norwegian citizens' tax funds are given to the Protestant Church until one registers as a member of another religious group, or as a member of the Humanist association.