In the exhibition catalogue, André Vauchez quotes GK Chesterton’s criticism of 19th-century biographers like Ernest Renan or Matthew Arnold who discounted Francis’s asceticism: “The moment Francis began to do something they did not understand or did not like, they did not try to understand, still less to like it they simply turned their backs.” St Francis is the saint (apart from those in the Bible) who is most widely represented in art, yet aspects of his life and message repel the dominant sensibility of one period or another. Indeed, he gave the gallery its first Italian primitive.

It isn’t small, being 5ft high, but William Coningham was no philistine when he protested. That was not enough for one 19th-century critic, who called the stupendous Zurbarán (confronting the visitor in the exhibition’s first room) of St Francis in Meditation clasping a skull, “a small, black, repulsive picture” when the National Gallery bought it, for £265, in 1853. They were optical instruments of religion, windows to another world. None of the older paintings here – the El Greco with palpable clouds resolving into a vision of Christ, or the great big Murillo of the saint embracing Christ on the Cross – were made to be seen in a museum. The coarse-spun garment is housed in a gilt Baroque frame, not because it is “art” but because it is a relic, to be viewed with devotion.

The object I found most moving in the National Gallery’s magnificent exhibition St Francis of Assisi was a sackcloth habit reputed to have been worn by the 13th-century poverello, the man who took poverty seriously.

St francis of assisi chesterton