Breivik isn't a Christian terrorist. He described himself as not being overly religious. He is also very critical of modern (I think that's what's meant by Western here) Christianity, both in the Church of Norway and Catholicism.
I've read somewhere that the Charlie Hebdo attackers weren't actually very religious either. Didn't visit mosques or read the Quran, but felt separated from society, and looking for some meaning to their life, some way to have impact, they found this. The article claimed they had more in common with Breivik and the Unabomber, or maybe even the Sandy Hook shooter and similar lone gunmen.
It wouldn't surprise me if the same is true for many extremists. ISIS also apparently gets a surprising number of recruits from Europe. People not fitting in looking for a place where they can be appreciated.
Wahhabism and other extremist versions of Islam have a significant following. Polls regularly indicate support or acceptance towards extremist views within Muslim communities. It's therefore not as simple.
