Ein feste Burg ist unser Gott

Christian Flor (1626 – 1697) was a German composer and organist. Working at churches in Rendsburg and Lüneburg, he was widely known for vocal and organ compositions. He composed one of the earliest Passion oratorios, in 1667. Johann Sebastian Bach probably became acquainted with compositions by Flor during his stay as a student in Lüneburgand may have been influenced by them. According to other sources, Bach is said to have known Flor personally and to have appreciated his compositions. His reputation is also supported by the fact that both Johann Gottfried Walther (1732) and Johann Mattheson wrote about him in their music encyclopaedias.
(source: Wikipedia)

This chorale prelude on the Luther hymn 'Ein feste Burg ist unser Gott' has been played on two manuals. You can hear the cantus firmus both in the right hand and the left hand. Within the settings for organ on this hymn (that I know of), this is quite the unique piece.