Almost 500 years ago this week, the Augustinian friar Martin Luther sent his 95 Theses to Albert of Brandenburg, the Archbishop of Mainz. Though no one – least of all Luther – realised it at the time, October 31, 1517 would be remembered forever after as the beginning of the Protestant Reformation.

Pope Francis has gone out of his way to commemorate this occasion. On Monday, he travelled to Sweden, where he marked the start of the Reformation’s 500th anniversary year with Lutheran leaders. This joint observation of an event that split the Western Church disturbs many Catholics. They are particularly unsettled by the idea of giving thanks for Luther, whose actions led (albeit unwittingly) to the death of millions in the European Wars of Religion.