Although it works well on the organ, this is not an organ piece but an harmony exercise. Previous pupils of Jean Gallon (former harmony teacher at Paris' Conservatoire) wrote each one such an exercise to celebrate Jean Gallon, and the book was edited.
Among them was Duruflé, who was Gallon's successor. This short and calm piece contains all Duruflé in few notes, with his genius to write beautiful modal harmonies.