Douglas Wilson is so good he has to be quoted in detail:
Take the vilest atheist you ever heard of. Imagine yourself sitting at his bedside shortly before he passes away. He says, following Sinatra, “I did it my way.” And then he adds, chuckling, “Got away with it too.” In our thought experiment, the one rule is that you must say something to him, and whatever you say, it must flow directly from your shared atheism—and it must challenge the morality of his choices. What can you possibly say? He did get away with it. There is a great deal of injustice behind him, which he perpetrated, and no justice in front of him. You have no basis for saying anything to him other than to point to your own set of personal prejudices and preferences. You mention this to him, and he shrugs. “Tomayto, tomahto.”
I am certainly willing to take the same thought experiment. I can imagine some pretty vile Christians, and if I couldn’t, I am sure you could help me. The difference between us is that I have a basis for condemning evil in its Christian guise. You have no basis for confronting evil in its atheist guise, or in its Christian guise, either. When you say that a certain practice is evil, you have to be prepared to tell us why it is evil. And this brings us to the last point—you make the first glimmer of an attempt to provide a basis for ethics.
You say in passing that ethical imperatives are “derived from innate human solidarity.” A host of difficult questions immediately arise, which is perhaps why atheists are generally so coy about trying to answer this question. Derived by whom? Is this derivation authoritative? Do the rest of us ever get to vote on which derivations represent true, innate human solidarity? Do we ever get to vote on the authorized derivers? On what basis is innate human solidarity authoritative? If someone rejects innate human solidarity, are they being evil, or are they just a mutation in the inevitable changes that the evolutionary process requires? What is the precise nature of human solidarity? What is easier to read, the book of Romans or innate human solidarity? Are there different denominations that read the book of innate human solidarity differently? Which one is right? Who says?