Keller believes that people all believe in moral obligation. Even though some may claim that moral values are subjective, we all behave as if some values are "absolute standards" by which to live. He gives the example of genocide that most would claim is wrong and evil. He says we have a sense that values are transcendent, an indication that God exists. Keller argues against evolutionary theory as an explanation for moral obligation. He maintains that altruistic behavior directed towards those outside one's group does not provide survival value, thus natural selection can not explain it. I believe even atheists like Dennett and Dawkins believe we have reached a point in our evolutionary development where we have transcended natural selection and are now in command of our future development. I don't think they would even argue that altruism toward enemies can be explained by means of natural selection.
This question of morality, of an inherent sense of good vs. evil, is one I wrestle with the most when I consider the question of God's existence. I understand that what each culture judges to be good or evil varies over time and place. Even the Bible, which many hold up as THE example of objective morality, displays a fair amount of subjectivity. However, one thing that is common (unless maybe you happen to be a psychopath) is that we have concepts of good and evil, love and hate. Are these just artificial constructs conceived of by the mind or are they independent of the human mind, rooted in reality and waiting to be discovered, like gravity?
I will end with a quote by Keller and invite readers to respond to it. "If you believe human rights are a reality, then it makes much more sense that God exists than that he does not. If you insist on a secular view of the world and yet you continue to pronounce some things right and some things wrong, then I hope you see the deep disharmony between the world your intellect has devised and the real world (and God) that your heart knows exists."