Saint Ambrose Barring Theodosius from Milan Cathedral (1620). Oil on canvas. 149 x 113. Saint Ambrose barring Theodosius from Milan Cathedral is a painting of c. by Anthony van Dyck in the National Gallery, London. It draws heavily on a 1618 treatment of the same subject by Peter Paul Rubens, on which van Dyck had worked as a studio assistant. In van Dyck's version, Theodosius is beardless, the architectural background is more defined, a spear and a halberd are added on the left-hand side and a dog in the bottom-left. The painting depicts the Roman emperor Theodosius I and his entourage being barred from Milan Cathedral by its archbishop Saint Ambrose, as punishment for the Massacre of Thessalonica.