Is there anyone who changed secular history more than Christ? Consider this: the followers of Jesus Christ introduced Gentiles to the God of Abraham, Isaac, and Jacob. Today, two out of five people in the world are Christians.

Likewise, where would civilization be today without Christian notions of compassion and solidarity? As atheists such as Bertrand Russell and Richard Rorty have noted, these ideas spring from the legacy of Christ. You do not have to be a Christian to appreciate parts of the legacy of Christ.

Five concepts worked out by Christian thinkers have especially affected modern ideas of politics and economics: human dignity, liberty and truth, conscience, and the notion of the person.

Human dignity: What is human dignity? The English word dignity is rooted in a Latin word meaning "worthy of esteem and honor, due a certain respect, of weighty importance." Both Aristotle and Plato held that most humans are by nature slavish and suitable only for slavery. Most do not have natures worthy of freedom. The Greeks used "dignity" for only the few, rather than for all human beings. By contrast, Christianity insisted that every single human is loved by the Creator, made in the Creator's image, and destined for eternal friendship and communion with him.

Among the figures of the Enlightenment, Immanuel Kant is probably the one who most clearly spoke to the concept of human dignity. He did so in the light of a categorical imperative that he discerned in the rational being, and he made famous this formulation of the principle of human dignity: "Act so that you treat humanity, whether in your own person or in that of another, always as an end and never as a means only."

It is not difficult to see in Kant's formula a statement in ...