Before we begin, we should note that insofar as belief in God makes better sense of various features of the universe, these features provide evidence for God. So the case for the existence of God does not depend on any one piece of evidence. No single piece of evidence, considered in isolation, proved that OJ Simpson killed Nicole Simpson and Ron Goldman. But the cumulative weight of all the evidence considered together pointed towards his guilt.
So, although morality provides enough evidence on its own to justify belief in God, here we’ll consider it as part of a cumulative case – something which should give the honest seeker additional reasons to believe in God. The evidence of morality works quite well when it is considered along with the case for the resurrection of Jesus and the design of the universe. So the seeker has some reason to believe in a personal creator, and to believe that Jesus rose from the dead.
First, we’ll note that logic does not dictate that the world had to contain objective moral values – and we can easily conceive of possible worlds which do not contain objective moral values. In fact, some atheists think that this world does not contain objective moral values. But the simple fact of the matter is that moral concepts have great explanatory power; furthermore the experience of morality is a central feature of human existence. Any worldview which attempts to explain morality away is deeply unconvincing.
Second, note that evolutionary theory does not explain morality away. Suppose one reason that we put our children’s well-being ahead of our own is because natural selection favoured those ancestors who had developed parental love. Does knowing why we feel that obligation mean that we don’t really have that obligation? Can we reasonably set aside our obligations to our children? Not at all. The social sciences could plausibly explain the rise and development of science. That would not mean that our scientific beliefs are false. Biologists explain why our perceptual systems developed. That does not mean that we should doubt our senses.
Third, there seems to be little room for objective morality in the universe of contemporary atheism: a universe which is merely a system of physical objects governed by mathematical rules. So some atheists attempt to identify objective moral values with “natural” states of affairs. The “good” might be identified with whatever benefits a species or a society.But these accounts do not seem very persuasive.
Suppose we discovered an intelligent, alien race, very much like our own. However, this species has survived and thrived by rape, cannibalism and other immensely cruel practices. For example, the stability of their society is founded on public displays of infanticide and the ritualised torture of innocents. Would we count these practices as “good” because they were beneficial? Even if it was demonstrated that their murderous and cruel activities allowed them to thrive in their environment, we would not say that their practices were “good”. For the universe would be a better place without such a species in it.
When we say that we should not engage in child torture because it is cruel, we do not merely mean that it is bad for society or that, overall, it leads to more pain than pleasure; we certainly do not mean that we spare children from suffering out of enlightened self-interest. We mean that even if the torture of children turned out to be beneficial for our family, our society, or even every member of every species we should not engage in it. It is not the sort of thing we should desire, no matter what the benefits were. So the good is not necessarily what benefits. Such natural states of affairs are not good candidates for moral values.
Furthermore, a state which is good for an individual or group only has objective moral value if the individual or group has objective moral value. States which promote the small pox virus are not valuable! States which promote the good of conscious, rational agents have objective moral value only if conscious rational agents have objective moral value. Suppose conscious rational agents are the outcomes of a random processes; and further suppose that conscious rational agents always pose a significant threat to other forms of life on their planet. Then conscious rational agents may not have objective moral value – and neither will states which promote their well-being.
Fourth, in contrast to atheism theists have a deep, persuasive account of “the good.” As a matter of brute fact, as the creator and sustainer of the universe God is the most important and valuable thing in existence. The earth and the opinions of human beings will pass away into the void; God’s values are eternal. God is supremely rational so his judgements can be trusted; he has fashioned our world and designed us. So every rational person should value what God values.
But God does not invent or create morality as we might create the rules of a new game. Moral values are not contrived by God; many flow logically from who he is and what he has created. Consider the value of human persons. We have immense significance because we are personal: we resemble a personal God. If he values himself (as a supremely rational ground of all existence would!) he will value us.
So the moral value of human persons simply follows from the fact that each human is important to God. The same goes for the value of our environment. If our world is an expression of God’s creativity we should place great value on it: a greater value than we place on profit or convenience. Furthermore, the creator made us to be like him; the values of kindness and compassion follow from this. So God did not invent moral values. Values follow from who he is and what he has decided to create. 
Fifth, if theism is true, then the moral life is also a rational life; it is not a long term exercise in self-harm. God is the guarantor of our trust in the value of goodness and virtue. God can meet our deepest desire for goodness, justice, forgiveness and love through eternal fellowship with him; conforming to God’s desires for us is not only good, but very much in our own interest.
Furthermore, if there is a God, then none of our moral projects are in vain. God created us to be good; when we are good we are fulfilling one of our purposes. We literally increase the number of valuable states in the universe when we are moral, because God values morality. If there is a God then, in the long run, there are no noble failures.
Finally, theism offers a neat explanation of moral obligation. We are not only aware of moral values but also moral rules which seem binding and inescapable. Why do we have an obligation to love others as much as ourselves? Why do we have a duty to be selfless? Why is it wrong to neglect these rules? Why do we feel shame when we break these rules – even when we are undetected? What can’t my own values – the value I place on my own interests – trump these pesky moral values? Where do these moral rules come from?
God has the knowledge, the character and the power to be the perfect moral authority. God would be the ideal observer. He would know all the long and short term consequences to our actions. He would not only be impartial; having unlimited love, God would earnestly desire the best for us. As creator God has the power to merit our respect; and as creator God has “property rights” over his entire creation. This explains why obligations exist and are binding: God’s instructions become our duties.
We do not need to know scripture to be aware of moral rules. Our consciences and moral intuitions may suffice when we reflect on them wisely. But note, our consciences impress rules on us; rules which are overriding and binding. It is almost as if conscience was attempting to communicate the commands of a law-giver. We feel shame when we fail morally; and we usually feel shame before a person. And duties and obligations are always to others, not to impersonal principles. Perhaps there is enough in moral experience alone to point us to God.
 Also note that we are not arguing that atheism will lead to morally chaotic or totalitarian societies; nor are we saying that atheist cannot be morally good. We do not believe that religion or revelation are necessary for moral knowledge or moral progress. The argument is merely that theism provides the best explanation for some important features of morality.
 Other atheists have argued that moral truths are what a hypothetical ideal observer, who is well-motivated, and who has all the relevant facts, would desire for each of us. So we simply imagine what this observer would want and that gets us a definition of the good. But why should we trust our moral imaginations if we cannot predict the future or know everything about the past and present? Won’t we just project our values onto this observer?
And why are the imagined desires of a hypothetical observer of any interest to us? We want moral values to guide our actions: but how do we know that virtues like courage and compassion, justice and mercy, won’t always be in conflict? Won’t there be a continual clash of incommensurable values? Won’t this leave us with a useless (or empty) definition of “the good”? And, in any case doesn’t this hypothetical observer sound a lot like, well, God?