Reading the Catechism of the Catholic Church, one is left with the impression that Catholic moral teaching is comprehensive, definite, and unchanging across the centuries. It is a comforting idea, but not an accurate one. As John Noonan recounts in detail in A Church That Can and Cannot Change (University of Notre Dame Press), the church’s moral teaching has evolved in a number of key areas, such as slavery, religious liberty, usury, and the dissolubility of marriage. The changes are significant. As Noonan notes, what was morally permitted (owning another person) became absolutely prohibited; what was endorsed as a moral and political good (burning heretics) came to be seen as a moral and political evil. What was prohibited as intrinsically wrong (lending money at interest) became permissible. And what was seen as impossible (papal dissolution of marriages where one or both of the parties is not baptized) became possible.

What are the ramifications of Noonan’s work for moral theology and ecclesiology? Catholics will be slow to grapple with those ramifications, I fear, because doing so will require us to confront three hard existential questions.

First, why did it take us so long to see the truth? According to the Venerable Bede, Pope Gregory the Great (590-604) decided to evangelize England when he came across some exotic white-skinned British boys for sale while browsing in the slave market in Rome. As Noonan notes, “Gregory’s heart-felt emotion is not distress at their enslavement by slavetraders, but at their enslavement to the devil.” It was not until fourteen hundred years later that the act of enslaving another human being was first declared by the magisterium to be intrinsically evil (by John Paul II in Veritatis splendor). The year before John Paul issued that encyclical, he visited the former “house of slaves” on the island of Gorée, Senegal, where he proclaimed, “It is fitting that there be confessed in all truth and humility this sin of man against man, this sin of man against God.” Noonan wryly observes, “What this confession did not remark was how recently the sin had been discovered.” How could Christians fail for so long to be moved by the pain, suffering, and degradation slavery inflicted on fellow human beings made in the image and likeness of God?

Second, what moral matters might we be wrong about today? To say that enslaving another person is an intrinsic evil is to say that it is always and everywhere objectively morally wrong. Yet, as horrifying as slavery seems to us now, it does not follow that those who failed to condemn it were wicked. In fact, some of them, like Gregory the Great, were saints. Nonetheless, they were trapped by the limitations of their own eras, their judgment impaired by unquestioned assumptions about the morality of certain inherited practices. What does that historical fact imply about us? Can we say for sure that we are not similarly trapped today? Can we say that there are no matters of enduring and grave moral import that have simply escaped our attention? Can we say that we ourselves will not be judged morally blind by our heirs in the faith?

Third, can moral teaching degenerate as well as develop? The notion of development suggests not only change, but also improvement. Doubtless most people will think that the changes in moral teaching that Noonan describes are not only welcome, but long overdue. Who today would doubt that religious liberty is morally preferable to justifying the burning of heretics? Yet depending on where one stands in history, doctrinal change can be for the worse as well as for the better. The church did not start out persecuting, or justifying persecution; rather, she was persecuted herself. Thanks in part to intellectual luminaries such as Augustine and Aquinas, Christian doctrine evolved-or rather devolved-over centuries to justify the use of force against heretics. How do we account for these substantial “wrong turns” our tradition took in its interpretation of the gospel? How do we distinguish legitimate development from potential “wrong turns” in our own time?

In dealing with these three questions we need to avoid both naive trust and hardened cynicism about the church’s capacity to perceive moral truth. We must also avoid the temptation to think that all problems of mistaken moral perception are in the past. Pope John Paul II apologized over and over again for sins committed by Catholics-on the watch of other popes. But it never seemed to occur to him that his own-and our own-capacity to perceive moral truths might be analogously limited by cultural and historical blinders. We need to keep searching for the appropriate balance of confidence and humility in the face of the lessons of history. Lumen gentium, Vatican II’s Dogmatic Constitution on the Church, articulates the problem in a nutshell: “The church...clasping sinners to her bosom, at once holy and always in need of purification, follows constantly the path of penance and renewal.”