Anybody who says that he is a "Christian Extremist". Basically, an extremist is somebody who takes the religion to the extreme and does acts like this, or bombing abortion clinics because they believe that God/Allah tells them to and will reward them in some specific way for their horrific acts. In this case, I have seen no evidence that he did this because he believed that God was going to reward him, or because he thought he was commanded to by his God.
He is an extremist, but it is of a different sort than "Christian"...
Basically this would be like saying that because he says he likes baseball he is therefore a "Baseball Extremist" and all people who like baseball are equally guilty. It's nonsense blather in the hopes of creating a visceral defense response from people in the group you select. I've read nearly half of his manifesto, so far I have seen no evidence that he was a religious extremist. I would have to say more of a political extremist from what I have read... a nutjob beyond a doubt. I don't believe his "cell" was other than him and the voices in his head.