Of course he is Christian. He is as much of a Christian as Osama bin Laden was a Muslim. Both of them were religious, both of their views were extreme, both were crazy, and both killed a bunch of people due to their close-mindedness.
What people fail to realize is that just because he is Christian, it doesn't mean all Christians are that way. Both sides need to remember that. Any time you generalize and label, you are probably wrong. I see some people jumping up and down acting like this is a victory against Christianity, which is stupid. Christianity is the result of the Crusades and the Inquisition. Do you really think one Norwegian nutjob is going to harm Christianity? Christianity has done more good than evil in my book, even if you don't believe in God. Then you have Christians loudly proclaiming that he was definitely not a Christian! How do you define Christian? Someone who obeys the bible? If that is the case, no one is Christian, since everyone lies and in several places the bible teaches opposing principles. You can't judge anyone to be a Christian beyond them saying they are or not. It is God's job to set them straight. Both sides just need to accept that this was a deranged little man with a gun and a bomb.