The existence of goodness can’t be explained. Our innate sense of right and wrong can only come from some non-natural source because good and bad, right and wrong don’t exist in the natural world. Love, kindness, generosity, compassion are all behaviors that are contrary to humans base, biological nature. So the fact that we can recognize them and the fact that they exist points to some higher power that must be the source.
These sorts of arguments have been repeated endlessly. Morality seems to be one of the rapidly shrinking gaps that believers insist must be occupied by a magical, invisible super being. But those gaps really are vanishing and there just won’t be anyplace left where we need to invoke God. The concept just won’t be of much use to us, and the liabilities associated with it won’t make it worth it to keep trying.
The morality arguments suffer from persistent circularity. The assumption from the outset seems to be that there is something magical and inexplicable about human morality. Then after some bluster, surprise, it is concluded that there is something magical and inexplicable about human morality.
A sober look at human moral behavior that doesn't presuppose that it is magical, or that there is an invisible, magical being, shows that there's a perfectly reasonable natural explanation.
The question has been addressed over and over in the literature, despite believers insisting that no such explanations exist. Richard Dawkins has written extensively about the evolutionary conditions that would select for virtuous traits in primates. See The God Delusion.
Frans De Waal has argued persuasively that altruism, compassion, emotional contagion, charity, and so on all appear in primates and their presence makes a positive contribution to survival. See Primates and Philosophers. and the Tanner Lectures
Steven Pinker, in the New York Times just a couple of weeks ago, gives a nice overview of the evolutionary foundations and explanations that we now have for human moral behavior. See The Moral Instinct.
The morality argument could pursue this strategy: they could argue that it is impossible in principle to explain human moral behavior in any other way than the existence of the Christian God, or even any old God. That is, it won't be enough to argue that none of the natural explanations are correct. The morality argument theist would need to argue that none of them, and none of the natural explanations we could ever come up with could ever explain the existence of human moral behavior. But that seems utterly implausible. Is the point that not a single one of the brilliant, world-class scientists who is offering natural explanations knows what he or she is talking about? Is there something about morality that is irreducible and inexplicable in natural terms? Even if they turn out to be mistaken, the accounts that are given in the sources above appear to be plausible, at least. Where’s the irreducible, magic part?
Arguments for the conclusion “X cannot be explained in principle by any natural account,” are always puzzling. Have all of the naturalized contenders been considered and rejected? Probably not. What sort of special access to some special non-natural facts does one have that resist explanation? It’s got to be more than a gut feeling or a deep conviction that morality is uniquely non-natural in the universe.
Religious believers have made those sorts of objections to natural explanations for centuries. And in countless cases, after they've insisted that X cannot possibly be explained by science, people who are earnest, curious, thoughtful and who weren’t willing to invoke God as soon as they encounter an intellectual challenge have come up with a correct, natural explanation of X. When a believer insists that “X is impossible to explain in any terms besides God,” and then are those industrious scientists who were making progress on X supposed to just stop going in to work? Should they just capitulate and say, “Ok, I’ll stop looking for any explanation except the Bible one you’re so fond of.” I’m really glad that they didn’t do that about polio, bubonic plague, lightening, drought, birth defects, ghosts, demon possession, the age of the universe, the age of the earth, the origin of life. Imagine the world we would be living in right now if scientists listened every time religious believers warned them that religious doctrine is the only way to explain the world.