Garry Wills is the author, most recently, of “Why Priests? A Failed Tradition.”

THERE is a poignant air, almost wistful, to electing a pope in the modern world. In a time of discredited monarchies, can this monarchy survive and be relevant? There is nostalgia for the assurances of the past, quaint in their charm, but trepidation over their survivability. In monarchies, change is supposed to come from the top, if it is to come at all. So people who want to alter things in Catholic life are told to wait for a new pope. Only he has the authority to make the changeless church change, but it is his authority that stands in the way of change.

Of course, the pope is no longer a worldly monarch. For centuries he was such a ruler, with all the resources of a medieval or Renaissance prince — realms, armies, prisons, spies, torturers. But in the 19th century, when his worldly territories were wrested away by Italy, Pope Pius IX lunged toward a compensatory moral monarchy.

In 1870, he elicited — from a Vatican council he called and controlled — the first formal declaration that a pope is infallible. From that point on, even when he was not making technically infallible statements, the pope was thought to be dealing in eternal truths. A gift for eternal truths is as dangerous as the gift of Midas’s touch. The pope cannot undo the eternal truths he has proclaimed....