It is both natural and unavoidable to describe this book, as the editors do, as the first systematic Protestant missiology. The Utrecht theologian Johannes Hoornbeeck held a series of disputations with students in 1662–4 on the subject of how Christianity might be taken to the gentiles. (Translating that word into English is not straightforward: the editors’ choice of ‘heathens’ is a sensible one, not least because the term Hoornbeeck would likely have chosen would be the Dutch heidenen.) Those disputations evolved into this hefty Latin treatise on the subject, whose English translation in this edition runs to 418 pages. It was eventually published in 1669, three years after Hoornbeeck's death.

He was not the first Protestant to discuss the subject, and in particular was influenced by his teacher Gisbertus Voetius, whose missiological writings predate Hoornbeeck's but were not published until 1676. But for Voetius and others missiology was a digression in a larger project, or was polemical, or a publicity or fundraising exercise. Hoornbeeck was the first to bring the full weight of seventeenth-century Reformed scholasticism to bear on missions, and the result is a strange hybrid of a text, a kind of theological monotreme. Its structure and preoccupations look distinctly weird to later eyes. And while its influence on its time and later generations was not zero, it was limited. But as a sign of how a leading academic mind was grappling with a formidably difficult problem, it is invaluable. We owe the editors a debt of gratitude for preparing such a clean, accessible text, carefully but not excessively annotated.

We might call this a work of missiology, but Hoornbeeck did not, since the category did not yet exist. He was feeling his way in trying to invent it. Book i (nine chapters) defines heathenism, and spends most of its scope surveying different heathens, ancient and modern. The remainder of the treatise, the fifteen chapters of book ii, devotes a great many pages to Christian arguments against heathen beliefs and practices, before turning in the final few chapters to what we might imagine is the core question: how the heathen might be converted. Some of the other questions he considers on the way are essential, notably his rejection of the claim advanced by contemporary Lutheran Orthodoxy that Christ's commission to preach to all nations had been given personally and exclusively to the Apostles, and had died with them. Some are distinctly not: the editors strive valiantly to explain why Hoornbeeck felt the need to include a twenty-five-page chapter rehearsing the conventional Protestant argument that the Sybilline Books were forgeries, but it still seems like an odd choice.

Still, that chapter serves as a clue to the coordinates of Hoornbeeck's mental world. Like any good Reformed scholastic, he felt more at home in the ancient world than in his own times. His ingenious determination to find ancient witnesses – both patristic and pagan – at every possible point of his argument is a reminder that, when early modern Europeans encountered non-Christian non-Europeans, they struggled mightily to see the people in front of them rather than the imagined ancient pagans whom they took to be a model for what gentiles should always and everywhere be.

Amongst the pervasive effects of this perspective is that Hoornbeeck thinks he knows how ancient Rome was converted, and so believes that the key weapon in the missionary's arsenal is the philosophical apologia. Lengthy chapters prove, using a mass of ancient sources and to his own entire satisfaction, that there is a single, creator God; that the soul is immortal; that the Christian revelation is self-evidently superior; that heathen worship is ridiculous; and that heathen practices are barbaric. He is aware that these arguments might have little purchase on alien peoples but struggles to do anything with that insight. When deploring the ‘animal nudity’ of indigenous Americans, for example, all he can say is that ‘because these people stray so far from reason and decorum in this respect, there are not many arguments that can prove them wrong … As a last resort they may have to be forced by authority and power’.

And so it is natural that he believes the peoples who will be easiest to convert will be the most civilised, in particular the Chinese and Japanese. Because such people already have extensive knowledge and great virtues, ‘they [will] more easily allow a new religion to be preached to them’. Whereas with non-literate peoples, there is a longer road to walk: ‘barbarians first of all should be made to learn to be humans, next to be Christians’. Subsequent centuries of missionary experience have shown these claims to be mistaken. In Hoornbeeck's world, they seemed self-evidently correct.

More surprisingly, his other principal reference point is contemporary Catholicism. He naturally deplores the brutalities of Spanish conquest and echoes the Black Legend, and makes clear his distaste for Catholic doctrine and practices. But he emphasises that the example of Catholic missions ought to ‘provoke us to emulate them’ (in contrast to some of his Lutheran contemporaries who claimed that Spain had proved that the entire missionary enterprise was corrupt and that Protestants ought to avoid it). Hoornbeeck not only cites Catholic authors – even papal decrees – at great length, affirming that ‘we are not at all ashamed to perceive and learn the good things that can be adapted for the benefit of the church’. He is also unabashed in his admiration, especially, of the Jesuits’ achievements and of their ‘conspicuous examples of virtue and sanctity’. Francis Xavier and Matteo Ricci are given particularly warm treatment, with even miracle-stories repeated uncritically.

This partly reflects the relative paucity of Protestant missionary examples on which Hoornbeeck might draw, although he was well aware of much more on that front that could have been said. The treatise's final, brief chapter is a cheerleading celebration of John Eliot's Native American mission in Massachusetts, carried out by ‘evangelists who not only teach the Gospel in a pure way with respect to doctrine, but also are saintly in their means and ends’. And yet this is an appendix to a Protestant volume most of whose authorities are ancient or contemporary Catholic writers. Hoornbeeck's explanation for such a generous – or omnivorous – policy is that, ‘like in a field full of all kinds of herbs, some healthy, some venomous, one should walk on tiptoe and the harmful things should carefully be separated from the good’. This is not only revealing of the entire worldview of Reformed scholasticism: it also remains a principle that other bitterly polarised eras might do well to consider.