The story of Saint Peter Martyr, a Dominican friar and inquisitor, is told in the 'Golden Legend'. In 1252, he was ambushed on the road to Milan by assassins hired by local Cathar heretics. He was wounded in the head and stabbed repeatedly. His last action, shown here, affirms his faith - he is writing 'credo' ('I believe'), in his own blood. Peter Martyr was recognised as a saint by the Pope. He became an important figure for Dominicans, ranking alongside the founder of the order, Saint Dominic.
X-rays show that the stance of Peter Martyr's murderer was changed from upright to stooping. Drawings made by Bellini for the new figure groups must have been reused as the basis of a workshop version at the Courtauld Institute, London. The four central figures of this work repeat exactly the outlines of those in the National Gallery picture. The Courtauld version once had the date 1509 on its reverse. The National Gallery picture, perhaps painted with assistance, probably dates earlier, to about 1507.
The woodsmen in the background chopping trees (which bleed in the Courtauld version) are intended to remind us of the way in which the saint was killed.