As some people discovered or already knew, the quotation in the previous post came from one of the first atheist manifestoes - at least within Western modernity - Baron D'Holbach's Le Bon Sens (Good Sense) (1772). I removed some distracting capitalisation from the version linked to in this post, but that aside, the thoughts are very modern. It's also worth noting that D'Holbach pulled no punches - if you dig into it, you'll find that he was scathing about the church and doctrines of the time.
When we say that there's nothing very new in the so-called "New Atheism", we're serious. There was, indeed, a publishing phenomenon, and it continues to some extent, but many of the actual arguments can be found right there in D'Holbach, as can the forthright attitude. In fact - no offence to anybody among the current New Atheists, but purely as a compliment to D'Holbach - D'Holbach's formulations of some of the points are as good as anything in the current literature. In any event, his work is an important part of our rationalist heritage and at minimum I thought it worth drawing attention to it.
As I think is clear, the passage I quoted does not provide a single knock-out reason that somehow makes moral rules rationally binding on us all, irrespective of whatever desires we might have. But he does offer powerful considerations that will move most of us most of the time to treat each other decently, and give us an incentive to create laws to cover the most important points where we cannot tolerate some people opting out.
Many thinkers, including some atheists, seem to want a more transcendent source for our moral codes, and for morality to be inescapably binding in reason. But I argue that we don't need anything so grandiose. The basic naturalistic basis has been available to reflective thinkers in all ages of human civilization. There's room for disagreement around the edges and for refinement as we discover more about ourselves, but similar rationales were offered by, say, the Epicureans in Hellenistic times and the Carvakas in India. Not surprisingly, David Hume said similar things to D'Holbach (so I'm not surprised that I've had suggestions that the passage might be from Hume).
It's when we look for more than these ordinary worldly reasons, which include our own responsiveness to each other, that we go wrong.