Who gets to define the word "Christian"? The three main sources would be: (1) Christian scriptures, (2) the Christian community, or (3) non-Christians. If we go with (1), Breivik fails miserably. If we go with (2), Breivik will be utterly rejected. Hence, the only way Breivik could be considered a Christian is if certain non-Christians (e.g. Muslims and the media) decide to define the term "Christian" as "anyone who calls himself a Christian, regardless of whether his beliefs line up with Christianity." Is this the route we want to go?
Since our Muslim friends really, really, really want to classify Breivik as a Christian, I have to ask a few questions. (i) Do non-Muslims get to define who is and who isn't a Muslim? (ii) Since the Lesbian author Irshad Manji calls herself a Muslim, is she a Muslim? (iii) How can Muslims reject Ahmadis as Muslims, but call Breivik a Christian, when Breivik obviously strays much further from orthodoxy than Ahmadis do?