He argues from reality and from the moral condition of human life. Dr. Schaeffer did this in his work among the disaffected youth who came to him and who had no confidence left, if ever they had any, in the Bible of the Christians. Ravi Zacharias used a similar form of argument in his magnificent lectures at Harvard a few years ago. C.S. Lewis was the past master of this form of argument. Here is Lewis discoursing on righteousness, self-control, and the judgment in Mere Christianity .
Atheism turns out to be too simple.
If the whole universe has no meaning, we should never have found out that it has no meaning. And it has one characteristic too remarkable to be ignored. He pointed out that all that would be required to damn anyone would be to hand a little tape recorder around his neck and demonstrate how often he condemned others for violating standards he violated himself. We are inescapably moral in our judgments, but utterly fail to live up to that morality we confess. And in his particularly winning way, he would point out that it is morality, after all, that makes life bearable and, finally, beautiful.