The media are describing Breivik, the Norwegian terrorist, as a Christian fundamentalist.
Fundamentalism has a specific theological meaning, and I doubt that Breivik has any strong opinions on the Virgin Birth or Substitutionary Atonement.
What the media seem to mean by a fundamentalist is a person with strong beliefs in a traditional religion of any sort, especially a person who takes those beliefs literally. But obviously a Thomist or a von Balthasarian can hold to Catholic doctrines strictly without thinking that Genesis describes 7 days of 24 hours each.
Zealot might be a better world – a Christian zealot, a Moslem zealot, a Jewish zealot, an environmentalist zealot, a Marxist zealot. Zeal is a virtue, but a minor one and one too easily disconnected from other virtues, such as justice and charity.
An even better term might be fanatic – that word conveys the disconnection from reality that terrorists usually manifest as well as a willingness to use violence on other people. It became a term of praise in Nazi Germany.