John Noonan returns here to issues he has considered at greater length elsewhere: slavery, usury, religious liberty, and divorce. His approach is forensic, a careful accumulation of evidence (especially in regard to slavery) which he then allows to speak for itself. What he wants us to hear it saying is clear enough: that Catholic moral teaching has changed significantly both in terms of now rejecting as intrinsically evil what it once considered morally acceptable (slavery) and of now regarding as acceptable what it once considered morally reprehensible (usury). He argues that the Church's teaching on religious liberty has changed between statements of Gregory XVI and Pius IX, and Vatican II's decree Dignitatis humanae. The twentieth-century application of what may or may not be called ‘the Petrine privilege’, by which the Popes dissolved non-Christian marriages in favour of the faith, is offered as an example of moral teaching in transition.

He begins, appropriately enough, with John Henry Newman, who in spite of his pioneering work on the idea of development in Christian doctrine could not accept that slavery was intrinsically evil, the terms in which John Paul II condemned it just a century later. Catholics engaged in slavery for centuries without any sense of sinfulness, Noonan says. It remained a ‘hidden sin’, a socio-economic and political institution towards which Christians felt powerless or indifferent. There were always dissenting voices as well as decisions that undermined slavery but Christendom for the best part of two thousand years accepted it.

Even in the Nineteenth Century, Catholics and Protestants, Enlightenment thinkers and religious orders all continued to ‘own’ slaves until the moral arguments of Quakers, Methodists, and evangelical Anglicans (with some Catholic abolitionists such as Daniel O’Connell thrown in), helped by diplomatic pressure not unconnected with British trade interests, finally (1839) persuaded Gregory XVI to condemn the slave trade as ‘inhuman’. Leo XIII and John Paul II carried this condemnation all the way so that the Church's teaching now is that slavery is intrinsically evil.

Developments in the other areas – usury, religious liberty, and divorce – are not so fully treated but the burden of these shorter accounts is the same: prima facie there has been significant change in Church teaching on all these issues.

Noonan presents the evidence without explicitly claiming too much. In the closing chapters he attempts a more systematic description of the process of change in moral teaching. Out of deeds comes law, he says, and out of difficulties comes development. Development cannot be denied but neither should it be exaggerated. One of the fears about ‘hidden sins’ is that we might ourselves come to be seen to have been perpetrators of actions that experience will later show to be evil: he suggests the use of the motor car (which kills a million people every year), the eating of animals, and the prison system, as things that future generations may come to regard as fundamentally immoral. But we are judged by the moral criteria we know even though experience has demonstrated the error of certain of the moral doctrines of our ancestors.

Following Newman, Noonan notes that development, whether doctrinal or moral, is a process of purification through which human beings grow intellectually, emotionally, socially, and morally. At the same time he believes that development in moral teaching is not susceptible to the kind of analysis to which Newman subjected doctrinal development. When development in moral doctrine is considered it must be remembered that in such matters the Church is teaching also by its deeds, he says, especially by its unchallenged practices and by the good conduct of spiritual persons. Experience has a role in changing moral teaching although he accepts that it is ultimately the Church that judges what experience counts. For the moment he is content to argue that there are different rules for the development of morals, and in any case Newman's checklist for doctrinal development is not the rule of faith. In fact in speaking of the rule of faith Augustine presents it as a moral rather than an intellectual criterion: the person whose interpretation builds up love of God and neighbour has understood the Word of God.

Noonan does not present arguments contrary to his own position: there is no defence attorney to respond to the prosecutor. He anticipates that people will want to argue against fundamental change in the Church's moral teaching but implies that this would be a fool's errand. Nevertheless it is a challenge that no less a thinker than Benedict XVI has taken on: in speaking to the Roman Curia at Christmas 2005 he addressed the issue of continuity and discontinuity in Church teaching. In modern times, he says, the Church was obliged to redefine three relationships: between faith and modern science, between the Church and the modern state, between the Church and the faith of Israel. In the process of such redefinition, Vatican II reconsidered ‘and even corrected’ certain historical decisions. But the discontinuity is only apparent, the Pope believes, for the correction of contingent decisions about contingent matters not only does not undermine the fundamental principles guiding the Church's teaching but actually helps those principles to stand out more clearly. In fact in its teaching on religious liberty Vatican II ‘returned to the most profound heritage of the Church’. John Courtney Murray, one of the chief architects of Dignitatis humanae, always stressed the distinction between religious liberty in relation to truth and religious liberty as a ‘civil right’.

Noonan's evidence is compelling. But further distinctions are required, not only of the kind used by Courtney Murray and more recently by the Pope, but also concerning levels of teaching authority within the Church. Ecclesiologists like Francis Sullivan have been engaged in painstaking work about the objects of infallible and of ordinary magisterial teaching and that work needs to be brought to bear on these issues also. With what level of authority is the Church entitled to teach on certain matters and with what level of authority has it done so? All Catholics will believe that the Church is entrusted with teaching people what is or is not relevant to their eternal salvation, and so the challenge presented by Noonan needs to be heard just as his presentation of the evidence needs to be complemented with a more systematic evaluation of its significance.