Those who do not believe that moral values must come from the Bible or be based upon G-d's moral instruction argue that they have a better source for values: human reason.
In fact, the era that began the modern Western assault on Judeo-Christian values is known as the Age of Reason. That age ushered in the modern secular era, a time when the men of "the Enlightenment" hoped they would be liberated from the superstitious shackles of religious faith and rely on reason alone. Reason, without G-d or the Bible, would guide them into an age of unprecedented moral greatness.
As it happened, the era following the decline of religion in Europe led not to unprecedented moral greatness, but to unprecedented cruelty, superstition, mass murder and genocide. But believers in reason without G-d remain unfazed. Secularists have ignored the vast amount of evidence showing that evil on a grand scale follows the decline of Judeo-Christian religion.
There are four primary problems with reason divorced from G-d as a guide to morality.
The first is that reason is amoral. Reason is only a tool and, therefore, can just as easily argue for evil as for good. If you want to achieve good, reason is immensely helpful; if you want to do evil, reason is immensely helpful. But reason alone cannot determine which you choose. It is sometimes rational to do what is wrong and sometimes rational to do what is right.
It is sheer nonsense nonsense believed by the G-dless that reason always suggests the good. Mother Teresa devoted her life to feeding and clothing the dying in Calcutta. Was this decision derived entirely from reason? To argue that it was derived from reason alone is to argue that every person whose actions are guided by reason will engage in similar self-sacrifice, and that anyone who doesn't live a Mother Teresa-like life is acting irrationally.
Did those non-Jews in Europe who risked their lives to save a Jew during the Holocaust act on the dictates of reason? In a lifetime of studying those rescuers' motives, I have never come across a single instance of an individual who saved Jews because of reason. In fact, it was irrational for any non-Jews to risk their lives to save Jews.
Another example of reason's incapacity to lead to moral conclusions: On virtually any vexing moral question, there is no such a thing as a [missing] purely rational viewpoint. What is the purely rational view on the morality of abortion? Of public nudity? Of the value of an animal versus that of a human? Of the war in Iraq? Of capital punishment for murder? On any of these issues, reason alone can argue effectively for almost any position. Therefore, what determines anyone's moral views are, among other things, his values and values are beyond reason alone (though one should be able to rationally explain and defend those values). If you value the human fetus, most abortions are immoral; if you only value the woman's view of the value of the fetus, all abortions are moral.
The second problem with reason alone as a moral guide is that we are incapable of morally functioning on the basis of reason alone. Our passions, psychology, values, beliefs, emotions and experiences all influence the ways in which even the most rational person determines what is moral and whether to act on it.
Third, the belief in reason alone is itself based on an irrational belief that people are basically good. You have to believe that people are basically good in order to believe that human reason will necessarily lead to moral conclusions.
Fourth, even when reason does lead to a moral conclusion, it in no way compels acting on that conclusion. Let's return to the example of the non-Jew in Nazi-occupied Europe. Imagine that a Jewish family knocks on his door, asking to be hidden. Imagine further that on rational grounds alone (though I cannot think of any), the non-Jew decides that the moral thing to do is hide the Jews. Will he act on this decision at the risk of his life? Not if reason alone guides him. People don't risk their lives for strangers on the basis of reason. They do so on the basis of faith faith in something that far transcends reason alone.
Does all this mean that reason is useless? G-d forbid. Reason and rational thought are among the hallmarks of humanity's potential greatness. But alone, reason is largely worthless in the greatest quest of all making human beings kinder and more decent. To accomplish that, G-d, a divinely revealed manual and reason are all necessary. And even then there are no guarantees.
But if you want a quick evaluation of where G-dless reason leads, look at the irrationality and moral confusion that permeate the embodiment of reason without G-d your local university.