The last part of this book is dominated by the effects of that discovery — not just the jolt it gave to abstract knowledge, but the practical effects of a process of conquest which brought Christian governors, and Christian priests, into close contact with real live pagans. Some parts of the resulting ferment of ideas are fairly well known, such as the great mid-16th-century disputation at Valladolid between the humanist scholar and pro-conquest hardliner Juan Ginés de Sepúlveda and the Dominican defender of the Amerindians, Bartolomé de las Casas. But Marenbon’s account sets them in a longer context of theological argument which few previous writers have considered in such depth. (Even so, Las Casas’s justification of human sacrifice may still take the reader by surprise.)

Marenbon’s survey of the 16th- and 17th-century debates may be a little more schematic than his searching account of the medieval arguments, but it does suffice to make one large point, which he emphasises in his conclusion: while we may think that the shift from medieval mentalities to early modern ones was a move away from rigid religious dogmas towards more human and tolerant positions, the evidence of these debates fails to support that view. There was no clear direction of “progress”, and the anti-pagan positions of some 17th-century Protestants and Jansenists were more uncompromising than those of almost any previous writers in the Augustinian tradition. Marenbon does not speculate about the reasons for this; one, surely, is the fact that “Socinianism”, from the late 16th century, and “Deism”, from the late 17th, were bugbears that genuinely frightened many mainstream theologians. Both were forms of “rational theology” (the former with a strongly biblical basis, at least to begin with, but the latter not even with that), with far-reaching implications about the power of human reason to work out what God would, or would not, do. The danger that the information provided by the Bible might turn out to be quite secondary (or even irrelevant) to human intuitions about the nature of divine justice now seemed very real, as it had never done before.

Which brings us back to revelation, and our liberal understanding of what is reasonable. It would be easy to read the story told in this book as a struggle between, on the one hand, people who were Augustinian because they were illiberal, and, on the other, their opponents, whose essentially liberal impulses drove them to find ways of accommodating virtuous pagans in the divine scheme of things. Such a portrayal would surely have been unrecognisable to the people involved. The idea that human ethical intuitions were primary, and that theological principles were secondary things, to be moulded to fit them, would have bewildered these thinkers. Today we live in a world where the expectations, and hence also the bewilderment, go in the opposite direction. That is, at the very least, another reason why we need an expert such as John Marenbon to guide us through the thinking of such a very different age.