I hate what appear to me to be bad arguments for Christianity - I always think it's such a bad mistake for apologists to get too excited about logical "proofs" for God that short-circuit actual belief and faith. The worst ones of course are the God of the Gaps arguments, that I have already discussed, which also touches on what is often called the "fine-tuning" argument for the existence of God. Before I look a little more into the moral argument for the existence of God with you, let me just spell out again why I think defending the faith in this way is most unhelpful - even destructive. The whole project and approach, in my view, is off base. The apologist often wants you to feel required - by logic and reasoning alone - to believe in God's existence (or Christ's resurrection, or whatever theological point is in view). Sometimes the more careful ones may employ might use words like "compelling", but even in using such vocabulary as this, two things are at play: the apologist believes, and belongs to a tribe that believes, that the arguments she presents are compelling... to her! Secondly, despite the relativistic language these rare, more careful apologists use, the posture and theological commitment behind the presentation belies the apparent care.
If you can assess these arguments critically as I attempt, and find them wanting, you might actually feel your faith is being undermined. That, at least, is my feeling in both the instance of the fine tuning argument and the moral argument (I do not have much to say in criticism of the cosmological argument, mainly because I take refuge in it as the others "fail" me). It is not that the arguments themselves are bad, although they could be framed quite differently, it is that their goals of proving God precede their presentation, and thus skew it. Rather, why not say: let's try and see how God could have given us morality! The apologist says: let me show you how God miraculously downloaded morality into human brains.
William Lane Craig for me is definitely one of the more careful apologists, and I enjoy his work. In this debate with Shelly Kagan, however, I find even this high-level debate just confirm the points I have just made. Here's the debate if you want to watch it, with a few comments below.
One of the views expounded by Kagan is "Contractarianism": perfectly rational beings would be able to come to an agreement about what the common rules needed to make a functional society. Or something like that.
Another naturalist view (although my point is of course not limited to naturalism): it is necessary that rational beings would reason about what is normal interactions between themselves.
Still others: where there is a command, there is a commander; where there is a law, there is a law-giver. Things don't come from nothing (a nod to the cosmological argument, really). But not necessarily. E.g. no-one needs to lay down a rule of non-contradiction for that to be necessary as a practice. But in morality, we could agree, says Kagan: the law-giver is all of us, says philosopher Shelly Kagan.
"Given the finality of death it really does not matter how you live", says William Lane Craig.
Only on theism can you make choices that are altruistic. Ultimately no difference to the heat-death of the universe.
Kagan was absolutely right to challenge Craig on the condemnation of holocaust being dependent on an ultimate and cosmic judge. However, he failed to point out an important part of the german regime's dogma. Firstly, those in charge did not really believe that they were doing something wrong. Secondly, the way that they justified this action to themselves and the rest of their tribe was to state that Jews were a harmful and lower race. Lions don't kill lions. Notice that in order for these atrocities to be performed it was absolutely necessary that in the Nazi mindset the victims really occupied a fundamentally different genus - a harmful genus.
Another thing that was not debated here was how civilisations slowly shift their moral perspectives. While it might seem just really obvious that murder is wrong, war is at least legal and soldiers are generally not considered guilty of murder. The same is true of abortion - although that depends on which country you're in. From a Christian perspective as well, there are stacks of laws that are no longer considered binding despite their presence in our holy book.
So I reject the moral argument as proof for the existence of God. Its starting point is not neutral enough, its goals affect its methodology, and the animal couter-examples are insufficiently accounted for. I happen to believe that morality is of God, but is not necessarily of God in the miraculous way Craig and others will try to present it. As a Christian, it is important to me to pray to ask God to help me and my family make good choices, to reflect back to God more and more my understanding of Christ's character in my own. Christ was not just "good". In fact, as the perfect man, he was what Adam was born to be: very good. That is to say: pleasing. I want my humanity to please God, but I haven't a cat in hell's chance without his help.