This choral is about the old Simeon after he saw Jesus at the Temple (Nunc dimittis).
Several comments tell that :
- the bass describes the old man's weariness
- the flow of the tenor is a a kind of remembrance of Simeon's life which is going to reach its end ("my course is run");
- the song expresses Simeon's serenity.
Anyway, it's probably one of the finest pieces from Orgelbuchlein.