Bach - Erbarm dich mein, o Herre Gott BWV 721

Even for a natural genius like Bach, it was not easy, as shown in this straightforward chorale arrangement ’Erbarm dich mein, o Herre Gott’ BWV 721 from ca. 1704. He set the melody in minims in the upper voice to an accompaniment of repeated four and five-voice chords in quavers. There is no more to it than that, which was enough for the great Bach connoisseur Albert Schweitzer to suggest in the early 20th century that such a simple piece could never have flowed from Bach’s pen.