Johann Bernhard Bach (1676 - 1749) was the second cousin of Johann Sebastian Bach (his grandfather was the eldest brother of the grandfather of Johann Sebastian Bach). He wrote several bicinia as choral preludes and they are invariably 'good'. The prelude to "Nun freut euch lieben Christen gemein" is no exception. The choral melody is played with the right hand, and the left hand plays the freely composed accompaniment. Bach uses fragments of the choral melody for the left hand part in short fore imitations to introduce each of the phrases of the choral melody. For the rest he writes free figurations in a fast paced rythmic texture, to lend contrast to the choral melody.
It's really a nice piece to play and with Advent nearing it can come in handy for any organist looking for something new te play.