In the choruses that open many of the Bach cantatas, the basic orchestral and choral setting is Baroque and flowery, but sometimes the whole movement has been built around a cantus firmus or 'fixed song', a simple melody that would have been old in Bach's time — occasionally reaching back to the epoch of plainsong. Many such cantus firmi appear in cantata's concluding chorales, but I find their setting within opening choruses more striking.
Hearing the ancient melody emerge from within the larger setting, if I am paying attention to the words and music, has the power to overwhelm me because it forces me to recalibrate my mind away from the tempo and organization of the rest of the setting. An ancient text of a few lines, sung in an ancient melody in long notes, seems to seize control of the rest of the choir and orchestra. It is like suddenly hearing a stern voice from the past in the midst of more frivolous chatter around you.
Striking examples include works at the edge of popular classical music, such as BWV 140 'Wachet auf, ruft uns die Stimme', as well as the German Magnificat (BWV 10 'Meine Seel erhebt den Herren'), BWV 78 'Jesu, der du meine Seele', BWV 26 'Ach wie flüchtig, ach wie nichtig', and the somber BWV 4 'Christ lag in Todes Banden'.
Albert Schweitzer left a fine essay (posted in a century-old translation on Aryeh Oron's magnificent Bach site, bach-cantatas.com)
After Bach, the bonds between the chorale and the sacred song are completely broken. The melodies that Emmanuel Bach, Johann Joachim Quantz, Johann Adam Hiller and Beethoven wrote, in artistic rivalry, to Gellert's poems only show what a distance separated them all from the chorale.
In the epoch of Rationalism, it is true, the melodies were not diluted to the same extent as the text; but there was still a hard struggle until the old melodies were again rehabilitated everywhere, and were no longer jostled in the chorale books by the characterless tunes of the later epoch. Now that this has been achieved, the dispute today is as to whether we shall retain the old chorales with the uniform note-values in which we have received them from the eighteenth century, or whether we should restore to them their original rhythmic variety. A definite decision, indeed, is hardly possible. Each "pro" that can be adduced from historical, artistic, or practical considerations is at once opposed by a "contra" of equal force in its way. Bach is concerned in this controversy to the extent that those who advocate the uniform polished form of chorales can plead that, although the opposite tradition had a powerful following all round him, he felt no artistic compulsion to revert to the old rhythmic form of the chorale, and so there is no cogent objection, from the purely musical point of view, against the chorale as we have received it from his hands. Against the enthusiasts for the rhythmic melodies the old master can plead as St. Paul once did against the Corinthians who knew all things much better, that he too thinks he is possessed by the spirit.
Neither flüchtig nor nichtig, in this case.