No one says atheists are incapable of doing things that a Christian worldview would say are virtuous, what the atheist really can't do though is give an account grounded within his own worldview for why you should do those things, or why it matters, or how many those things would have ever been considered "virtuous" in the first place had Christianity never existed given that much of those things weren't before then and in many cases still aren't even today in societies that were never Christian, and that's the difference.