As a good Christian, however, Abelard thought that only belief in the incarnated Christ could bring salvation; so he also supposed that where an ancient pagan thinker had tiptoed towards this threshold of Christian belief, God had then stepped in to bestow, by supernatural means, some prophetic knowledge of Christ’s human existence on earth.

In this way Abelard supplied later writers with not one but two very fertile ideas: the notion that valid theological knowledge did circulate among ancient pagans, and the claim that people could be turned, by a “special inspiration” from God, into Christians, long before the actual coming of Christ. (As a theoretical possibility that last idea had already been put forward by Augustine himself, whose “City of God” existed, interspersed among the human race, in all ages; but Abelard’s argument that wise pagans had reached the very threshold of Christian belief by their own efforts was deeply un-Augustinian.)

The most influential opponent of Augustine was Thomas Aquinas. His answer to the question of whether pagans could acquire real wisdom was a resounding “yes”: the towering philosophical structure which he spent a lifetime building had the teachings of Aristotle as its foundations, and the nature of the construction was meant to demonstrate a seamless transition, above a certain level, from the truths of human philosophy to the ones supplied by divine revelation.

On the issue of pagan virtue, Aquinas respected the theological principle that virtue in the full sense must be animated by the love of God, but he dismissed the Augustinian idea that pagans cannot be virtuous at all: they can indeed do “those good works for which the good of nature suffices”.

As for salvation: on this point Aquinas seems most radical of all, but, as Marenbon shows, he was simply developing a line of thought set out by previous writers. While he supposed that God might use “special inspiration” in exceptional cases (including Kaspar Hauser-like children, brought up among wolves without any human instruction), for his general solution to the problem he turned to a quite different concept: “implicit faith”. Pagan philosophers who had arrived at a basic monotheistic understanding could vow to believe whatever might be known about God by those whose knowledge was greater than theirs. In the ancient world, those superior figures were in fact the Jewish prophets, illuminated by God. But it was not necessary for pagans to meet them, or even to know who they were; a sincere belief that such people must exist was quite sufficient. The flexibility — or, if you prefer, generosity — of this argument is rather breathtaking.

All these positions, pro- and anti-Augustinian, were adopted primarily in order to argue about pagans of the ancient world, especially the most virtuous Greeks and Romans. Any interaction between these debates and discussions of contemporary pagans was quite limited, thanks to the continuing belief that, in the words of St Jerome, “no people remains which does not know the name of Jesus, and, even if they have not had a preacher, they cannot however be unaware of the faith from neighbouring peoples”. The discovery of the New World would shake that assumption to its foundations.