There's a fascinating debate here between John Hare and Peter Singer, in which the latter opens by giving all the standard arguments why no belief in God is necessary in order to have a theory of morality. I won't rehash all the arguments. What I thought was fascinating was the point Singer conceded: the atheist has difficulty fully motivating a commitment to the moral life. The problem naturally arises once your ethical system makes high demands on you. Singer is a utilitarian, which, once you work it out, makes rather enormous demands.
What I found to be an amazing tension in his discourse was between his statement, on the one hand, that nothing could be more important than knowing how to live morally and, on the other hand, his answer to a personal question posed during Q&A. When asked how he dealt with moral failure in his own life, he revealed that he did not, in fact, feel guilt, but merely accepted that he is not as good as he ought to be. I suppose he also implied that he would try to improve bit by bit, which is fair enough. On the other hand, he was quick to add that, to be sure, he already gave more of his income than most people who call themselves Christian. It was truly striking to hear such an answer, coming as I do from a tradition which acknowledges human frailty, our constant need of forgiveness, and the need for humility.
The experience highlighted for me one common point of tension in these debates over God and morality. It is now a standard litany among Christian intellectuals debating the topic that "of course, atheists can be moral, too, and indeed some of them are so moral as to put most Christians to shame." All the same, we clearly do not quite agree on what really is right and wrong. So it's hard to know by what standard we're repeating this litany.
But that is not the point which stuck out most. Listening to Singer, I was left with a profound question: is morality nothing more than an abstract concept? It's important to mention that Singer (like Hare) emphasizes the central role of reason in determining right and wrong, and more particularly the necessity of taking the most universal point of view possible. By considering the consequences of our actions not only for ourselves and those around us but also for every other conscious creature in existence (an abstract form of the Golden Rule), we come to increase our ethical knowledge.
It is not the enormity of that task which particularly bothers me. Rather, the product of this process seems detached from reality. We get a theoretical vision of what might maximize something called happiness or utility or well-being. But the vision is ahistorical--it doesn't go anywhere. Commitment to this vision is entirely optional, as Singer concedes. If one commits to the ethical life, it is not in hope of any fulfillment, but merely because one is inclined to follow abstract principles wherever they lead.
The Christian vision of ethics, it seems to me, is entirely different. The point of Christian ethics is not to obtain a theoretical blueprint of how to maximize an abstract quantity, like happiness. The point is to be part of God's ongoing project of saving the world. Thus our commitment to live morally or immorally is not an arbitrary choice, but instead a response to a commitment that God has already made--we are either with him or against him. When we ask what it means to live ethically in the world, the question is not how to maximize an abstract quantity, but rather what will it look like to live in the world that God will one day realize.
Of course, the Christian vision is incomprehensible if one thinks of God himself as abstract and impersonal. Then one is left, as Singer is, with no plausible answer to the problem of evil. If God is defined by the three qualities of omniscience, omnipotence, and omnibenevolence, then God does not exist. Yet I thank God that such a god does not exist, because if it did, I do not see how anything in the world would ever happen--everything would be predetermined by a set of principles, and the kind of spontaneity that characterizes all that is living and beautiful would not exist.
For the Christian, God is not known through abstract principles which define him, but through God's story. He, like us, has a history, which starts with glorious creation but also involves the pain and suffering not only if his creatures but also his own suffering and death. If the resurrection of Jesus tells Christians that God's victory over evil has finally begun, this does not mean that the victory will be easy or that it won't be messy.
Now, if Jesus really didn't rise from the dead, none of this really matters. But I think there are compelling arguments to suggest he did, and as a bonus I find it to be a much firmer grounding for ethical thinking than secular rationalism. I certainly don't begrudge anyone who wants to try and live morally for whatever reason. I simply find that when I contemplate the choice to make moral commitments, I can't help but feel paralyzed at the thought of how arbitrary such a choice would ultimately be--unless that choice is in response to a project which is already at work in the world. In other words, I have a hard time with morality in the abstract. The world is either going somewhere or it isn't, and if it isn't going anywhere, then I don't see much point in doing anything.