“Wachet auf, ruft uns die Stimme” BWV 645 is part of Bach’s Orgelbüchlein (Little Organ Book), which is a collection of chorale preludes for the organ. This particular chorale prelude is based on the hymn of the same name by Philipp Nicolai. The hymn itself is associated with the Advent season and is often used in church services.
The piece is known for its expressive and contemplative character, and it has been appreciated for its intricate counterpoint and harmonic richness. It’s a beautiful example of Bach’s skill in adapting and expanding upon existing hymn melodies in his compositions for the organ.