Joe Carter outlines three arguments for the existence of a transcendent moral lawgiver at his blog The Evangelical Outpost. The three are all variations of the same argument - that there must be an objective ground for morality if moral discourse is to make any sense. The versions Carter summarizes are those of Immanuel Kant, C.S. Lewis, and D. Elton Trueblood.
The arguments of these thinkers are not intended to serve as proofs of the existence of God, but they are certainly powerful pointers in that direction. Unless one adopts the position of moral nihilism it's very difficult to evade the force of the reasoning, and the move to adopt nihilism is, of course, a move to forfeit rationality.
Read Carter's summary and see if you don't agree that the moral experience of humans all but demands an either/or choice between nihilism and the existence of a transcendent moral authority.RLC