Why did Bach’s chorale arrangements use familiar hymn tunes instead of newly composed melodies?

In the context of the Lutheran church, Johann Sebastian Bach utilized familiar hymn tunes in his chorale arrangements because these melodies were central to the Protestant liturgy and were intended to be sung by the congregation alongside the choir.

Rather than composing original melodies, Bach focused his artistry on creating elaborate, rich harmonic settings of these well-known tunes. By using established hymns, he ensured that the music remained accessible and participatory, as the congregation was expected to join in the singing. These chorales served as a fundamental element of his passions and cantatas, where the familiar melody provided a recognizable foundation for his sophisticated contrapuntal and harmonic work.1