A textbook "moment" of Bach -- in a charming setting of the three verses of the German song "O Lamb of God, Most Holy," suddenly near the end of the third verse Bach finally heeds the text and shows us the strange despair we are praying for mercy to avoid. He employs several musical devices in this sudden moment: a change in meter, a suggestion of a distant tonality, and a barrage of harsh chromaticism (notes outside of the key).

First we learn the background and the tune "O Lamm Gottes, unschuldig" before showing how Bach introduces this Cantus Firmus (melody) in this organ prelude's beginning and first verse where it is heard on top. The Cantus Firmus moves lower in the second verse, and in the third it is down at the bottom in the organist's pedalboard. Here the text of the last line changes from "have mercy on us" to "grant us peace." Ending strong and firm, Bach gives us peace from that sudden harsh "moment."

Netherlands Bach Society: "O Lamm Gottes, unschuldig" as played by Wolfgang Zerer for All of Bach

Thank you to listener David for the excellent suggestion.