In Alex Rosenberg's The Atheist's Guide to Reality the author unabashedly promotes a scientistic view of knowledge. "Scientistic" does not mean "scientific," rather it describes a view based on "scientism" which is the view that science is the only guide to truth about the world and human existence. If a claim cannot be demonstrated empirically, using the tools of science, then it's not something that we can know, and in fact is not something we should even believe.
In Rosenberg's view physics "fixes all the facts" about what is and what can be reasonably believed. This is sometimes called "physicalism."
Not all scientists are scientistic or "physicalists," many of them hold that there are truths about the world that science is not equipped to discover, truths about justice, rights, beauty, and morality, for example, but Rosenberg thinks it's neither good science nor good epistemology to hold beliefs about these things.
Rosenberg is no dummy. He's the chairman of the philosophy department at Duke University and demonstrates in his book a considerable breadth of learning. He also strives to be rigorously consistent. Given his belief that physicalism is the correct way to understand reality it follows that there is no God, no miracles, no soul or mind, no self, no real meaning or purpose to life, no meaning to history, no human rights or value, no objective moral duties - only what he calls a "nice nihilism."
By "nice nihilism" Rosenberg means that nature has fortuitously evolved in us a tendency to treat each other well despite the fact that doing so is neither a moral duty nor morally "right." That, for the one who embraces Rosenberg's scientism, is the only glimmer of light in an unrelentingly dark world and even this little glimmer is beset with problems.
Here's one: If our niceness is the product of impersonal undirected processes then it cannot have any moral purchase on us. That is, it can be neither right nor wrong to be "nice." Some people are and some aren't, and that's the end of the matter.
Evolution has also evolved behaviors that are not "nice." If we're the product of evolution then there's really no way to morally discriminate between "niceness" and rape, torture, lying, racism, etc. All of these behaviors have evolved in our species just as niceness has and we have no basis for saying that we have a moral duty to avoid some behaviors and embrace others.
In other words, on scientism, there are no moral obligations and nothing which is wrong to do.
Rosenberg admits all this, but he thinks that we need to bravely and honestly face up to these consequences of adopting a scientistic worldview and a scientistic worldview, in his mind, is the only intelligent option in a world in which there is no God.
I think he's right about this, actually, and argue in both of my novels In the Absence of God and Bridging the Abyss (See the links at the top of the page) that the consequences outlined in The Atheist's Guide to Reality do indeed follow from atheism. The atheist who lives as if none of these consequences exist is living out an irrational delusion, most likely because he can't live consistently with the logical and existential entailments of what he believes about God.
A belief or a worldview that entails conclusions one cannot live with, however, stands in serious need of reexamination.