This painting is part of the group: Two Panels from an Altarpiece
This painting and 'Saint Paul', to judge by the perspective, were perhaps originally part of the upper tier of an altarpiece in compartments (polyptych). Since the pictures are Milanese, the bishop is likely to be Saint Ambrose who was the patron saint of Milan.
Other paintings in this group: Two Panels from an Altarpiece
The National Gallery, Trafalgar Square, London WC2N 5DN