Dec. 29, 2010  -- He's bashed the Catholic church as "an institution more concerned with preventing contraception than child rape" and compared Islam to a "death cult."

These and other fervent attacks on religion have earned writer and scientist Sam Harris enough acrimony that Harris -- the author of the bestselling "The End of Faith" -- now travels with security guards and keeps his home address under wraps.

But here's the twist: It's not just the faithful who are furious with Harris -- it's also his fellow atheists.

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Harris first raised their ire at an Atheist Alliance International convention in 2007, where he argued that atheists shouldn't actually call themselves atheists. (CLICK HERE to watch an excerpt of the speech.)

"It seems to me that we are consenting to be viewed as a crank sub-culture. We are consenting to viewed as a marginal interest group that meets in hotel ballrooms," he said.

Harris instead emphasized the importance of advocating for "reason and intellectual honesty" and to "destroy bad ideas wherever we find them," later adding "that religion has more than its fair share of bad ideas."

Harris also made a more surprising argument: That, despite his views on religion, he believes it's possible for people to have real and positive "spiritual" experiences.

"If we by definition ignore them because of their entanglement with religion, we appear less wise than even our craziest religious opponents," he said.

Harris, as it turns out, spent his 20s studying meditation and Eastern spirituality in India and Nepal and elsewhere. After the Sept. 11 terrorist attacks, he felt compelled to start writing books that rail against religion. But, to this day, he continues his own spiritual practice.

This practice, he said, "gives me better reason to attack religion. Because I know that the religious belief, the dogmatism, the faith, is unnecessary. I know you don't need faith. I know you don't need to pretend things you don't know in order to investigate spiritual experience or to have spiritual experience."

Now Harris is angering fellow atheists once again, this time with a new book called "The Moral Landscape." In the book, Harris, who has a Ph.D. in neuroscience, launches a full frontal attack on his fellow scientists, faulting them for turning responsibility for the debate over morality to religious people.

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Scientists, he argues, should be the arbiters of morality.

"Science has basically divorced itself from the most important questions," he said. "Questions like: How to raise children? What constitutes a good life? When you should help people and how much? The most important questions. What diseases should we cure first? What wars are worth fighting? How many refugees should we let across the border?"

Scientists and scholars have been tearing into his book -- criticism that Harris appears to take more seriously than that levied by religious believers.

"When an evangelical writes to me with lots of capital letters saying, 'You don't understand, the cave was empty on the third day,' I don't have to burn a lot of fuel wondering if I've read the Bible closely enough. And maybe I am completely wrong about the resurrection," he said. "But when a well-known philosopher or well-known theoretical physicist writes me and says, 'Listen, you are completely wrong and here's why,' ... it merits a little more attention to see whether or not he or she has actually discovered something wrong in my argument."

Watch the full story on "Nightline" tonight at 11:35 p.m.