(Breivik) Getty Images (L to R) Bill O'Reilly and Anders Breivik.

Ever since the murderous rampage in Norway last week, Bill O’Reilly and other American conservatives have sought to insist that the alleged Norwegian terrorist Anders Breivik is not a Christian. “Breivik is not a Christian,” O’Reilly said on his Fox News show on Tuesday. “That’s impossible. No one believing in Jesus commits mass murder.” Elsewhere on the channel, Fox contributor Ralph Peters argued that “[Breivik] defines himself as a Christian, but you know, anybody can claim anything.” And conservative Christian activist Jordan Sekulow wrote at the Washington Post that “to label Breivik a ‘Christian’ requires a depraved understanding of what it means to be a Christian.”

The belief underlying these denials of Breivik’s professed Christian identity is that news outlets would only label a terrorist “Christian” in order to discredit Christianity. News coverage of the Norway was initially muddied when some outlets inaccurately called Breivik a “Christian fundamentalist.” A more accurate description would have been Christian nationalist. But the Christian designation itself was informed by Breivik’s own voluminous writings, which show a man animated by a very twisted version of Christian thought and history.

In fact, the clash-of-civilizations nature of Breivik’s manifesto is not so different from the warped interpretation of Islamic thought and history that motivates some Muslim terrorists. Mainstream Islam does not call for the destruction of non-Muslims or require Muslims to establish a global Islamic state. And yet when Muslims try to insist that terrorists who subscribe to such extremist beliefs do not represent true Islam, O’Reilly and his compatriots dismiss those arguments out of hand.

During last summer’s controversy over whether an Islamic center should be built in lower Manhattan, O’Reilly criticized “liberal media” for “buying into the genteel Islam.” Other Fox News figures have falsely claimed that Muslims never condemn violent acts by other Muslims, such as Fort Hood shooter Nidal Hasan. Glenn Beck rarely misses a chance to conflate ordinary Muslims with terrorists, saying on his radio show last year: “After you’ve killed 3,000 people, you’re going to now build your mosque on – there, really?”

O’Reilly remains unchastened by the fact that he routinely rebukes Muslims for distancing their faith from terrorists in the way that he has sought to distance Christianity from Breivik this week. He and other conservatives have focused on their doubts about whether Breivik “practices” Christianity as reason to refrain from labeling him Christian.

But there’s a reason the Bible urges Christians to avoid judging the faith of others. It’s a tricky business. Does Christian faith derive from church attendance? Bristol Palin recently told Christianity Today that her family doesn’t really go to church these days, and they’re hardly alone among Americans who call themselves Christian. Does Christian faith adhere to one narrow definition? Sekulow argues that Breivik cannot be Christian because his manifesto states that he does “not necessarily have a personal relationship with Jesus Christ.” While that’s a key aspect of evangelical Christianity, plenty of other Christians don’t consider their faith incumbent upon a personal relationship with God.

The hubris involved in judging another’s faith takes on added relevance during election season. Bill Clinton’s critics were often quick to charge that his weekly church attendance was “showy” and that his sexual misconduct proved that his faith was fraudulent. (They must have skipped over the part about how we have all sinned and fallen short of the glory of God.) In 2004, conservative Catholics declared that John Kerry was not a “real” Catholic because of his support for abortion rights.

Just this month, several Republican candidates have disputed Barack Obama’s Christianity, including Scott D’Amboise, who is challenging Senator Olympia Snowe in Maine. “I don’t believe he’s a Christian,” D’Amboise told David Frum. The Maine Republican apparently didn’t get the memo that the preferred way to question Obama’s faith is with the formulation, “Well, he says he’s a Christian.”

I have no interest in defending Anders Breivik or in making a brief for his Christian identification. But I’ve always found the eagerness to judge another’s faith offensive. I was told countless times that I was not actually a Christian when I went on conservative talk radio to promote a book about religion and politics several years ago. Like many Americans, I’ve had the experience of sitting in a pew and being told that it’s not possible to be a Christian and vote for Democratic candidates. But most upsetting was the episode a decade ago when a friend asked me to coffee so we could talk. I went straight from church to the Christian coffee shop, only to have her tell me that she didn’t think I was sufficiently Christian and was worried that I hadn’t really been saved.

O’Reilly and others do damage to Christianity by implying that it’s so fragile as to be threatened when one member does something truly vile. As heinous as Breivik’s alleged acts of terror were, Christianity has withstood worse. Islam has withstood the past ten years of turmoil within and criticism without. The best defense against terrorists like Breivik is to show how his interpretation of Christianity is wrong, not to deny that he could have come out of the tradition. Have a little faith, Mr. O’Reilly.

Amy Sullivan is a contributing writer at TIME, and author of the book The Party Faithful: How and Why Democrats are Closing the God Gap (Scribner, 2008). Articles of Faith, her column on the intersection of religion and politics, appears on TIME.com every Friday.