was he a Christian?

He was a much a Christian as most hard right conservatives, with a willingness to take direct action one of the only differences. from Sullivan:

What does "practice" Christianity mean? Are the only Christians church-goers? Do you have to go once? Or weekly? Is O'Reilly himself a Christian by his own definition? And, of course, the "Christian angle" did not just come from a cop. It came from the manifesto of the mass murderer himself. And here is the mass murderer's own definition of Christianity, also from yesterday's Dish:

"As this is a cultural war, our definition of being a Christian does not necessarily constitute that you are required to have a personal relationship with God or Jesus ... Being a Christian can mean many things; That you believe in and want to protect Europe's Christian cultural heritage. The European cultural heritage, our norms (moral codes and social structures included), our traditions and our modern political systems are based on Christianity – Protestantism, Catholicism, Orthodox Christianity and the legacy of the European enlightenment (reason is the primary source and legitimacy for authority). It is not required that you have a personal relationship with God or Jesus in order to fight for our Christian cultural heritage and the European way."

This, to point out the obvious, this is straight out of the Fox News playbook. It is orthodoxy in the current GOP. It is in no way more extreme than what Hannity and O'Reilly and Beck argue day after day after day. Indeed, the killer's obsession with the "war on Christmas" is less intense than Bill O'Reilly's. And here is O'Reilly's definition of Christianity in the same segment, a definition so close to Breivik's it could almost be the same person writing:

The second reason the liberal media is pushing the Christian angle is they don't like Christians very much because we are too judgmental. Many Christians oppose abortion. Gay marriage and legalized narcotics, secular left causes. The media understands the opposition is often based on religion. So they want to diminish Christianity and highlighting so-called Christian-based terror is a way to do that.

Notice that O'Reilly defines Christianity in entirely political terms related to the control of other people's lives and bodies. i.e. being judgmental in passing laws to restrict the freedoms of others for the greater good. It is straight out of the school of thought I described at length in "The Conservative Soul." In other words, O'Reilly's definition of Christianity is very close to Breivik's. Both are best understood as Christianists, who see Christianity primarily as a way to change or mold civil society and the lives of others for what they see as the greater good, but O'Reilly is a non-violent one who deplores violence, while Breivik takes his own rhetoric so seriously he felt obliged to destroy Norway's civil order in order to save it.
The difference is not in ideology, but in the move to violence. That move is, of course, a central, profound and vital one, and O'Reilly's views of the world are in no way responsible for what just happened in Norway. But it is hard to see where O'Reilly would disagree with vast tracts of Breivik's ideology - except the resort to violence. Ideologically, there is scarcely any difference at all.