Because of his strict views on predestination, we tend to think of Calvin as representing the hardest part of the hard-line Reformation. Yet, as Gordon shows, he spent a huge amount of time trying to promote and sustain compromise positions — above all, on the central question of the Eucharist, where the Lutheran doctrine of the Real Presence of Christ was flatly denied by the Zwinglians. Against the odds, Calvin formed lasting relationships both with Luther's disciple Melanchthon, and with the leader of the Zwinglians in Zurich, Heinrich Bullinger: diplomacy, even deliberate ambiguity, played a part here, but so too did a capacity for friendship, loyalty and trust.

These delicate manoeuvrings between friends and doctrinal rivals fill many pages of this book. But of course they are not the whole story. Today, everyone has heard of Calvinism, while no one talks about Bullingerism, and Melanchthonism is discussed only by academic specialists. Gordon explains the ingredients of Calvin's lasting success: he provided the essential tool-kit for reformed Christianity (a classic doctrinal handbook, a catechism, a system of church government), and his Geneva Academy turned out hundreds of ardent young ministers to take those tools elsewhere and apply them.

This is an impressive book, well-balanced in the sense that it is not at all partisan (the burning of Servetus is put in context, in a way that may partly exculpate Calvin, but other incidents show that he could indeed be irascible, even vindictive). Yet something is missing. Perhaps Bruce Gordon, a professor at the Yale Divinity School, was afraid that too much doctrinal detail might be a turn-off for the general reader. Whatever the reason, there are moments in this book — key turning-points, in fact — where the actual contents of Calvin's beliefs are left frustratingly vague.

His conversion to Protestantism is described in terms so general that only a brief reference to "idolatry" gives any clear idea of why he may have felt compelled to reject the Catholic Church. When Gordon summarises the teachings of Calvin's great handbook of reformed theology, the Institutes of the Christian Religion, almost every phrase he uses ("There is no disharmony between God's revelation and the order of creation", etc) could just as well be applied to Aquinas's Summa theologiae. And I doubt whether any reader could work out what the Lutheran doctrine of the ubiquity of Christ's body might be, on the basis of Gordon's compressed and fleeting explanation.

Beza also said rather little about Calvin's core doctrines in his own biography of him. But that was because he lived and breathed them every day. His final defence of Calvin's notorious temper was that he was fired with zeal to defend essential truths about God. Whether or not Gordon agrees with that view, it would help if he said a little more about what Calvin believed those essential truths to be.