Those who do not believe that moral values must come from the Bible or be based upon God'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 God 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 God 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 God 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 godless -- 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.