Morality is Common
There are certain emotions and concepts that science simply can't explain. Our fundamental moral nature is one of them. How then are these rules explained? Are they the products of random gene selection or of a divine God? That moral rules are accidental is an argument that falls flat on the evidence of a common design. In other words, the basic moral laws that exist (against murder, stealing, lying, etc.) are common among all men.
Conclusion: A Moral Lawgiver
Common moral laws indicate a moral Lawgiver with the power to implant these laws in mankind. And this fact - that we have a moral conscience, or the sense of morality - may be the most persuasive of all the arguments for God's existence. The very values that are woven through the human fabric, driving us to a better world and pricking our conscience when violated, validate the great moral Designer who created us.
An interesting corollary to the argument by morality is the Argument by Universal Consent. Stated simply, mankind as a whole has at all times and everywhere believed, and continues to believe, in the existence of some superior being or beings on whom the material world and man himself are dependent. This fact cannot be accounted for except by admitting that this belief is true, or at least contains a germ of truth. Admittedly Polytheism, Dualism, Pantheism, and other understandings of this being have mingled with and disfigured this universal belief of mankind, but this does not destroy the force of the argument about existence. It only begs for a better definition of the nature of the being we are considering.