The story can be briefly told. In 1572, the Protestant Henri sought to unite war-torn France by marrying the Catholic Marguerite of Valois. All went reasonably well until his mother-in-law, Catherine de Medici, ordered the murder of the Huguenots gathered for the nuptials. 6,000 were slaughtered on St Bartholomew's Night, 24 August. Henri converted, until it was safe to return to the Protestant faith.
Two decades later, faced with a continuing war, Henri conceded that a peaceful France must be a Catholic France. His enemies attributed to him the cynical phrase "Paris is worth a mass". They mistook his real faith, which was above all in France. Henri was perhaps Europe's first great post-Christian prince.
For Henri had a larger unifying scheme, a proposal to unite Europe in a grand Christian republic, whose emperor would be elected by the rulers of the European nations. The chief obstacle to this was Habsburg power, and it was while preparing for an assault on the Empire in 1610 that Henri was assassinated.
His compatriots tend to admire him as clever, brave, sardonic, sceptical and craggily attractive. His nickname points up an irreducible difference between the English and the French. "Le vert galant" just doesn't mean the same as "Ladies' Man".
Anglo-French antipathy was as fundamental a European theme in Henri's time as in ours. His great counsellor Sully reported from James I's court: "The English hate us; it is undoubtedly an effect of their arrogance and pride, for no nation in Europe is more haughty and insolent, nor more conceited of its superior excellence."
Plus a change. What fascinates is the difference between Henri's aspirations for a united France, and for a united Europe. He saw that France could be united only as a Catholic state; but that French, German, Spanish, English and Swedish Europe could be united only if confession and nation were surrendered to some larger cause. Such a Europe would be inevitably post-Christian, for any attempt to harmonise the conflicting European confessions inevitably resulted in yet another Christian division.
Henri's vision has taken nearly four centuries to be fulfilled. The European states have consolidated themselves inside borders often remarkably like those of his time; even the religious patchwork of Catholic, Calvinist and Lutheran is much the same now as then.
So what do Christians pray for in such a post-Christian Europe? Each of our nations is gathered within a culture of consent, determined largely by language and custom. It is within those boundaries that we have found it possible to be most fully at ease with ourselves, and not much further outside. To pray for more is to ask for too much.
But what about beyond those boundaries? If there is a consensus within the Union, it is at kind of Euroscepticism; that the "Europe des Patries" dreamed of by Henri is about as far as we can go, and to dream beyond that is to invoke the nightmare of empire. To gather conventionally around a single currency is as arbitrary and limiting as to gather around a single confession, and just as likely to end in chaos. And we already have a single currency; it exists in cyberspace, along with the Peseta and Barings, and its local value changes by the nanosecond.
Further, Europe is already too big and too linguistically diverse to be a single polity in a conventional sense. But just as Henri was stuck with a Catholic France, so we are stuck for the moment in the clumsy, grey botch we call the European Union. It is worth looking forward to something different.
Europe's future union has to be both bolder and more subtle. It has to be both Gospel and Church. Its Good News must be one of a shared humanity. Its Church must be a complex of institutions that obtains consent to the patterns of shared humanity that we want, and adopts the compulsions that we thereby need. But our history should warn us; the writ of those consents, and compulsions, will not run far beyond the boundaries of each nation state.
Clearly a wider Europe is now possible; to pray merely for a Union of 16 is to ask for too little. It is exciting to imagine a civilised and peaceable Europe from Reykjavik to Vladivostok. Here is a goal worthy of our dreams and capable of our achievement, if we deploy Henri's great quality - to know when to settle for something less, and when to reach for something greater.