Luther has gone down in history as the man who shattered the unity of Western Christendom. But he was reluctant to leave the Catholic Church, and if he returned today he would find that many of the reforms he proposed have come to pass

Here is one for the next parish hall quiz night. When did Martin Luther leave the Catholic Church? As we limber up to mark the 500th anniversary of his Ninety-five Theses (though evidence is thin that he actually nailed them, as legend has it, to the door of the castle church in Wittenberg on 31 October 1517), some may answer that the date is obvious.

The Ninety-five Theses were not just a protest against the sale of indulgences and other corrupt practices in the Church but a rejection of papal authority. So their appearance, nailed or not, surely marked the moment when Luther stopped being a Catholic (and an Augustinian friar) and struck out on his own. Formally, though, the correct date should be 3 January 1521, when Rome finally acted on the threat of excommunication it had made the previous year in the bull, Exsurge Domine (which Luther had publicly burnt at the gates of Wittenberg, in revenge, he said, for the torching of his own books, on the Church’s orders).

But is excommunication the last word? Those thus punished are ex communio – “out of communion” – with fellow Catholics, but because their baptism can never be revoked, they are, of course, still Christians. And, just as it inflicts the punishment, so the Church can lift it. Official teaching stresses that excommunication is a “medicinal” imposition, designed to cure mistaken beliefs and bring about repentance.